Assessment the effects of Anthropogenic activities on sediment physico-chemical characteristics of Owalla Reservoir, Southwest, Nigeria

doi:10.21203/rs.3.rs-2492095/v1

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Assessment the effects of Anthropogenic activities on sediment physico-chemical characteristics of Owalla Reservoir, Southwest, Nigeria

https://doi.org/10.21203/rs.3.rs-2492095/v1

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The study assessed the physico-chemical characteristics of sediments from Owella reservoir in Osun State, Nigeria with the primary aim of creating a baseline information in aspect of limnology and also to further assess influenced by anthropogenic activities as well as characteristics that were affected mostly by natural sources in the water body. Twenty sampling stations were established along the reservoir representing its three different sections and two zones for a period of two years. On the average, the reservoir sediment was mostly sand, clayey-sand in texture, acidic in chemical reaction, low in nutrients and moderate in organic matter. Although, nutrients were identified to be mostly of anthropogenic origin, while alkali metals and alkaline earth metals were from both anthropogenic and natural sources. The overall order of dominance of the exchangeable cations was: Mg²⁺ > Ca²⁺ > H⁺ > Al³⁺ > K⁺ >Na⁺ and anions in the order PO₃⁴- > SO₄²⁻ > NO₃⁻> Cl⁻. Significantly difference (p < 0.05) were observed in the physico-chemical characteristics of the sediment across the two zones (littoral and open water) of the reservoir and horizontal variation along the three basin sections (upstream, mid-basin and downstream) and over the two seasons of the year were mostly not significant (p > 0.05). The overall mean values obtained for particle size distribution along the reservoir were in the following order: sand (7.31%) > Silt (43%) > clay (57%). Surface run-offs are major contaminant in Nigeria waterbodies and through geochemical weathering of the top soils. The results showed low concentration of cations, anions and nutrients compound in the sediment of Owella reservoir which suggests low influence of anthropogenic activities on the sediment quality.

Reservoir

sediment

anthropogenic activities

physico-chemical

contaminant

Sediments, which include all detrital, inorganic, or organic particles that eventually settle at the bottom of waterbodies (such as lakes, ponds, rivers, streams, bays, estuaries, brackish, and oceans), play a crucial role in an aquatic environment. Typically, they are made of materials from both natural and anthropogenic sources. Sediments are typically a matrix of materials that can be very heterogeneous in terms of their physical, chemical, and biological features, and they can operate as both a sink and a source of nutrients and/or other contaminants in waterbodies (Power and Chapman, 1992). Sediment is made up of organic matter and weathered elements, and it varies from place to place depending on the parent materials, climate, and biological activity (Boyd, 1995). They are made up of a variety of elements, such as clay minerals, carbonates, quartz, feldspar, and organic solids, and are produced from a variety of material inputs from various sources, such as eroded rocks and soils, waste materials, atmospheric fall-out, and inorganic materials produced biologically (Power and Chapman, 1992). Typically, sediments are divided into two useful categories based on the size of their constituent particles: coarse (sand) and fine (silts and clays). The majority of the coarse fraction is made up of inorganic silicate minerals that are stable, non-cohesive, and typically free of chemical contamination.

The fine fraction is made up of particles with a relatively high surface area to volume ratio and frequently have surface electric charges, making them more chemically and biologically reactive than coarser sands and increasing the possibility of sorption and desorption of pollutants (Dirisu et al. 2017). Most frequently, chemical contamination is seen in the fine ("muddy") sediments that make up depositional zones. The environment's elements and the sediments' inherent properties, such as water movement, pH, organic carbon load, sediment movement, pore water residence duration, and water chemistry, all have an impact on how successfully the sediments adsorb pollutants. The most crucial aspect influencing sediment type is water quality, and sediment's physical and chemical characteristics have an impact on how well it can bind toxins (McGlathery et al., 1994; Koch et al., 2001). Because they have a higher surface area to volume ratio, sediments with smaller particle sizes will absorb more pollutants (Simpson et al., 2005). However, sediment quality may also be reflected in water quality (Power and Chapman, 1992). The physical properties of sediments include the sediment profile, textural structure, particle size, temperature, aeration, and color. Sand, clay, and silt make up the particle size. But of the three, clay is the most active because of its electrical charge, substantial surface area, and presence of organic matter (Boyd, 1995). According to the relative numbers of these particles, sand, silt, and clay, the texture of sediment is defined as the distribution of the various soil particle sizes or the proportion of each (Wild, 1993). (U.S.EPA, 1995). However, the chemical features of the sediment could be categorized as organic matter, acidity, alkalinity, gases, oxidation-reduction potential (ORP), pH, conductivity, ion exchange capacity, cations, anions, nutrients, and organic carbon (Brady and Weil, 2002). The primary locations for microbial decomposition of organic detritus and biogeochemical recycling of nutrients, chemicals, and elements are lake sediments (Wetzel, 2001). Surface and deeper sediments may be mixed by physical and biological processes, and previously buried sediments may be exposed at the surface (Kennett, 1982). Sediments may be remobilized and moved to new areas after initial deposition by waves, currents, or other movement mechanisms. Chemicals that are present in the solid phase naturally or artificially may be redistributed into flowing water by physical processes and organisms (Kennett,1982).

This study's goal was to assess the Owalla Reservoir's sediment quality, the geographical and temporal fluctuations in its physico-chemical properties, and to offer useful data on the trend in pollution levels from anthropogenic activities on the sediment quality.

Study area

The Owalla dam is located at Oba-Oke close to Okinni in Osogbo, Osun State and it lies between Latitudes of 07⁰ 53.5'N − 07⁰ 59.0'N and Longitudes of 004⁰ 31.5'E to 004⁰ 35.0'E with an average elevation of 336 ± 8m above mean sea level as presented in Fig. 1. The dam had been constructed as a reservoir for the Ede Power Plant by the Water Corporation of Nigeria. The reservoir is bordered by the Odo Otin, Ifelodun, Irepodun, and Orolu Local Government Areas of Osun State at its northern, eastern, western, and southern portions, respectively. The reservoir has a maximum width of 3.5 kilometres and a maximum length of 12 kilometres from the dam wall to the upstream (i.e., the point where the main river, Erinle, enters the reservoir). The dam wall is approximately 677 m long and 27.5 m high. Its surface is around 14.5 km², and its volume is approximately 94 106 m³ (Aduwo and Adeniyi, 2018). The reservoir was designed to improve the quality of water suppy to about 7 cities in Osun State (Osogbo, Ede, Ife, Gbongan, Erin-Osun, Ilobu and Ifon) as well as other towns and rural communities in Osun central, Osun West and Ife area in Osun state (Abdu-Salam et al., 2013). The study area is of lowland tropical rainforest flora makes up the landscape (Keay, 1959; Agboola, 1979), the soils are ferruginous tropical red soil (laterites), with little erosion and soil deterioration (Smyth and Montgomery, 1962), typical tropical climate and is surrounded by the basement complex of southwest Nigeria (Aduwo and Adeniyi, 2018). Domestic waste and agricultural operations in the catchment basin are the main anthropogenic sources of contaminants in the area. The major activities going on around this reservoir are: farmers, fishermen, or traders of agricultural products. The reservoir has great potential to become a major tourists’ attraction, in the state due possibilities of engaging in various water sports and activities. Neighbouring communities in the area, commute via canoes to get their children to school and other domestic activities.

Sampling Stations and Program

Twenty (20) sampling stations were established on Owalla reservoir representing the entire three portions of the reservoir (upstream, mid-basin, and downstream) and two zones (littoral and open water), (Fig. 1 and Table 1). Bottom sediments were collected every three months for period of two year in a plank boat propelled by an outboard motor every three months.

Table 1

Grid Co-ordinates and Morphometric Characteristics of the Sampling Stations
S/N	Grid co-ordinate		Elevation (m)	Distance & Direction from dam (km)	Depth range (m)	Mean depth ± S.E (m)
	Latitude (N)	Longitude (E)					Description of Station
	Latitude (N)	Longitude (E)					Reach	Region
1.	07^o 58.644 ^Ꞌ	004^o 32.889 ^Ꞌ	336 ± 7	9.26 (N)	0.98–4.24	3.04 ± 0.57	Upper Reach	Open Water
2.	07⁰ 57.575 ^Ꞌ	004⁰ 33.115 ^Ꞌ	332 ± 9	7.47 (NE)	3.93–7.05	6.05 ± 0.60	Upper Reach	Open Water
3.	07⁰ 57.506 ^Ꞌ	004⁰ 32.903 ^Ꞌ	332 ± 8	7.42 (N)	1.20–4.00	2.88 ± 0.48	Upper Reach	Littoral
4.	07⁰ 57.673 ^Ꞌ	004⁰ 33.287 ^Ꞌ	332 ± 8	7.55 (NE)	2.00–5.53	3.88 ± 0.57	Upper Reach	Littoral
5.	07^o 56.515'	004^o 34.361'	336 ± 7	5.98 (NE)	2.71–6.81	4.76 ± 0.73	Mid-Basin	Open Water
6.	07^o 56.664'	004^o 33.758 ^Ꞌ	334 ± 7	5.96 (NE)	4.15–9.60	6.40 ± 1.06	Mid-Basin	Open Water
7.	07⁰ 56.867'	004⁰ 33.741'	335 ± 7	6.29 (NE)	2.36–5.18	4.24 ± 0.52	Mid-Basin	Littoral
8.	07⁰ 56.518 ^Ꞌ	004⁰ 33.685 ^Ꞌ	334 ± 8	5.71 (NE)	1.98–4.77	3.41 ± 0.47	Mid-Basin	Littoral
9.	07⁰ 55.611 ^Ꞌ	004⁰ 32.699 ^Ꞌ	340 ± 7	4.98 (N)	2.31–5.56	4.90 ± 0.43	Mid-Basin	Littoral
10.	07⁰ 55.636 ^Ꞌ	004⁰ 33.044 ^Ꞌ	334 ± 7	4.51 (NE)	4.07–6.73	5.58 ± 0.47	Mid-Basin	Open Water
11.	07⁰ 55.760 ^Ꞌ	004⁰ 33.449 ^Ꞌ	334 ± 7	4.91 (NE)	2.25–5.68	4.19 ± 0.60	Mid-Basin	Littoral
12.	07⁰ 54.831 ^Ꞌ	004⁰ 33.886 ^Ꞌ	338 ± 8	2.99 (NE)	6.90–9.12	8.06 ± 0.44	Lower Reach	Open Water
13.	07⁰ 55.005'	004⁰ 33.956 ^Ꞌ	336 ± 8	3.23 (NE)	1.73–4.94	3.34 ± 0.69	Lower Reach	Littoral
14.	07⁰ 54.732'	004⁰ 33.813'	338 ± 7	2.88 (NE)	1.00–3.50	2.49 ± 0.53	Lower Reach	Littoral
15.	07⁰ 54.206'	004⁰ 32.147'	339 ± 7	1.94 (N)	2.65–5.31	3.65 ± 0.44	Lower Reach	Littoral
16.	07^o 54.089¹	004^o 32.403¹	341 ± 7	0.87 (N)	12.70–17.26	14.25 ± 0.81	Lower Reach	Open Water
17.	07⁰ 53.905'	004⁰ 32.689'	339 ± 7	1.56 (NE)	3.16–5.89	5.01 ± 0.63	Lower Reach	Littoral
18.	07⁰ 56.594'	004⁰ 33.487'	333 ± 8	5.71 (NE)	0.18–0.25	0.21 ± 0.01	Mid-Basin	Littoral
19.	07⁰ 54.802'	004⁰ 33.460'	336 ± 8	2.81 (NE)	0.15–0.20	0.17 ± 0.01	Lower Reach	Littoral
20.	07⁰ 53.669'	004⁰ 32.792'	342 ± 10	1.10 (E)	0.16–0.20	0.18 ± 0.01	Lower Reach	Littoral

Sample Collections/Field Determination and Laboratory Analysis

The bottom sediments were collected using a Van-Veen Grab with a 0.04 m² area (0.2 m x 0.2 m) to ascertain the physico-chemical properties of Owalla reservoir. The samples were gathered and stored in a clearly labelled polythene bag for laboratory examination while the elements in the sediments were analysed using air-dried method (i.e. samples were dried at room temperature in sterile trays). To ensure accurate results and reliability of the data gathered using acceptable standard methods and procedures that is adequate and proper quality control and quality assurance (QA/QC) measures were observed both at the field site and during the laboratory analysis. Particle size analysis was determined using the hydrometer method with sodium hexametaphosphate {Na₆(PO₃)₆} as dispersing agent (I.I.T.A., 1999) and the textural composition was determined based on the percentage occurrence of the three main sediment particles (sand, silt and clay) using the triangular graph method (Shepard, 1954). Hydrogen-ion concentration (pH) was determine using a glass electrode pH meter (PCE-PHD-1 pH meter) oxidation-reduction potential in 1: 2 sediment-water suspensions were also assessed after 30 minutes of equilibration (Blakemore et al. 1987). Na⁺, K⁺, Mg²⁺, and Ca²⁺ are exchangeable cations, and 1N (pH 7.0) ammonium acetate solution, NH4OAC, was used to extract these metals from the sediment and dissolve them in water (I.I.T.A., 1999). The Na⁺, K⁺, Ca²⁺ and Mg²⁺ concentrations were measured using a flame analyzer at wavelengths of 295 nm, 383 nm, 285 nm and 211 nm, respectively, while Mg²⁺ and Ca²⁺ were measured using Flame Atomic Absorption Spectrophotometer (Perkin-Elmer Model 403) respectively (UAE University Manual, 2013).

Exchangeable acidity (H⁺ and Al³⁺): The sediment's contents were extracted into an aqueous solution using 1.0 M KCl, and their concentrations were determined by titrating the filtered extract with 0.01 M NaOH and HCl solutions while using phenolphthalein as a colour indicator (University of Stirling, 2004). Based on a sediment-to-water suspension of 1:5, chloride (Cl⁻) was calculated. This suspension was shaken for an hour in a shaker before being filtered under suction using Whatman No. 1 filter paper (i.e. using a suction pump). Titrimetry was used to determine the filtrate's chloride level using a 5% potassium chromate (K2CrO4) solution and 0.014N silver nitrate solution (APHA et al., 1995). Using 1N calcium acetate Ca (OAc)₂, sulphate (SO₄²⁻) was extracted from the sediment, and the concentration was then quantified spectrophotometrically (turbidometric technique) using the violet filter at 420 nm wavelength (APHA et al., 1995). Using 4.4 ml of the digestion mixture, 0.2 g of sediment was digested for 2 hours to determine the total nitrogen levels of the sediments (digestion mixture contains Selenium powder, Lithium sulphate and Hydrogen peroxide). The mixture was then given 50 cc of water, which was thoroughly incorporated. It was produced up to 100 ml after chilling, carefully mixed, and then left to settle. To each 0.1 ml clear supernatant of the standards and samples, 5 ml of reagent comprising sodium salicylate, sodium citrate, sodium tartarate, and sodium nitroprusside was added, thoroughly mixed, and then left to cool for 15 minutes. After adding 5 ml of a solution of sodium hypochlorite, water, and sodium hydroxide to the standard and samples in a test tube, it was left for an hour to allow for color development. After that, a spectrophotometer was used to measure the absorbance at 665 nm (Anderson and Ingram, 1993). Using (0.03N NH₄F + 0.025N HCl), phosphorus was recovered from the air-dried sediment using the Bray N0-1 method. The phosphorus concentrations were then measured using a 660 nm colorimeter (I.I.T.A., 1999). By utilizing a mixture of acidified potassium dichromate (10 ml of 1N K₂Cr₂O₇), or chromic acid as an oxidant, to digest a known quantity (between 0.1 and 0.5 g) of air-dried sediment for a full 24 hours, the organic carbon contents were determined titrimetrically. Barium-diphenylamine-sulfonate (BDAS), an indicator, and ferrous ammonium sulphate (FeNH₄S0₄.7H₂0), a titrant, were both used to titrate a blank sample (of distilled water) (Walkley and Black, 1934).

Statistical Analysis

The data obtained were analysed by computing the descriptive statistics, Analysis of Variance (ANOVA) to determine sediment physico-chemial quality variations among the stations using Microsoft Excel, and Paleontological Statistics (PAST Version 3.17) software

The results of 29 sediment physico-chemical characteristics of Owella reservoir, determined including particle size distribution were discussed in Tables 1, 2, 3, 4 and 5 respectively. Table 2 showing the overall sediment physico-chemical properties of Owella reservoir. The overall depth value ranged between 0.98–21.0 m with mean value of 5.17 ± 0.29 m. The mean values recorded for transparency, air, water and sediment temperature during the study periods were 1.56 ± 0.04 m; 36.0°C; 33.0°C and 34.0°C, respectively. Highest mean percentage of particle size was observed for sand (7.31 ± 1.66%); Silt (43 ± 9.51%) and clay (57 ± 1.07%). Textural types shown by some ternary plot are 100% sandy, silty-sand (4%), sandy-clay (3%), sandy-mud (3%), clayey-mud (2%) while silty-clay, silty-mud; clayey-silt and sandy-silt were 1% each of the total sediment textural type as presented in Fig. 2. The order of dominance of the exchangeable cations Mg²⁺ > Ca²⁺ > H⁺ > Al³⁺ > K⁺ > Na⁺ and anions in this order PO₃⁴- > SO₄²⁻> NO₃⁻ > Cl⁻ was obtained during this study. Overall mean values recorded for pH (in H₂O), pH (CaCl₂), ORP (in H₂O) and ORP (CaCl₂) were 5.27 ± 0.06; 4.80 ± 0.05; 85.26 ± 3.54mV and 111.47 ± 2.85mV. Conductivity value observed in this study ranged from 64.3–2630 µScm^− 1 with mean value of 284.98 ± 45.65 µScm^− 1. The overall mean concentrations for aluminium, hydrogen ion, sodium, potassium, calcium and magnesium were 0.28 ± 0.01 cmol/kg; 0.04 ± 0.20 cmol/kg; 0.18 ± 0.01 cmol/kg; 0.28 ± 0.02 cmol/kg; 1.45 ± 0.10 cmol/kg and 2.14 ± 0.09 cmol/kg, respectively. Overall mean concentration recorded for chloride, sulphate, phosphorous, total nitrogen and organic matter were 31.46 ± 1.05 µgg^− 1; 172.56 ± 18.31 µgg^− 1; 97.78 ± 7.36 µgg^− 1; 0.13 ± 0.003% and 3.20 ± 0.20%. Nitrate concentration ranged from 35.44–9746 with mean concentration of 5354.76 ± 144.02 µgg^− 1 while 19.293–1368.85 range was recorded for phosphate with mean value of 342.71 ± 26.69µgg^− 1. Among the physico-chemical parameters, silt and sulphate recorded highest coefficient of variation (CV) > 100%; for clay, conductivity, ORP (in H₂O), aluminium, hydrogen ion, sodium, potassium, calcium, magnesium, CEC, total nitrogen, phosphate and organic matter: 50% > CV < 100%. Transparency, air temperature, water temperature, sediment temperature, sand, pH (in H₂O), pH (CaCl₂), ORP (CaCl₂), base saturation, chloride,nitrate and phosphorus recorded CV < 50%.

Table 2

Overall descriptive statistics of physico-chemical characteristics of sediment from Owalla Reservoir
S/N	Parameter	Unit	Descriptive Statistics
S/N	Parameter	Unit	N	Min.	Max.	Mean	Median	S.E.M	S.D.	Skewness	Kurtosis	%CV
Hydro-physical and Meteorological Parameter
1	Depth	m	136	0.98	21.00	5.17	4.44	0.29	3.33	2.12	5.71	64.4
2	Transparency	m	136	0.58	2.53	1.56	1.59	0.04	0.41	-0.10	-0.58	26.1
3	Air Temperature	^oC	152	24.4	36.0	30.0	29.8	0.24	2.93	0.11	-1.15	9.8
4	Water Temperature	^oC	152	27.0	33.0	29.8	29.5	0.12	1.47	0.42	-0.55	4.9
5	Sediment Temperature	^oC	152	24.0	34.0	27.5	27.0	0.13	1.64	0.97	1.85	5.9
Sediment Physical Characteristics
6	Sand	%	152	15	99	73.1	81.5	1.66	20.45	-0.99	0.002	28.0
7	Silt	%	152	0	43	9.4	6	0.77	9.51	1.40	1.89	100.9
8	Clay	%	152	1	57	17.4	12.5	1.07	13.23	1.17	0.48	76.0
Sediment General Chemical Characteristics
10	pH (in H₂O)		152	3.53	6.82	5.27	5.28	0.06	0.76	0.01	-0.63	14.3
11	pH (CaCl₂)		152	3.47	6.58	4.80	4.77	0.05	0.60	0.19	-0.31	12.4
12	Conductivity	µScm^− 1	152	64.3	2630	584.98	411	45.65	562.84	1.65	2.35	96.2
13	ORP (in H₂O)	mV	152	-5	186	85.26	84	3.54	43.59	0.01	-0.64	51.1
14	ORP (CaCl₂)	mV	152	9	190	111.47	114	2.85	35.11	-0.16	-0.37	31.5
Sediment Exchangeable Cations
15	Aluminum ion (Al³⁺)	cmol/kg	152	0.10	1.40	0.28	0.25	0.01	0.17	3.20	14.13	60.8
16	Hydrogen ion (H⁺)	cmol/kg	152	0.15	1.30	0.40	0.35	0.02	0.21	1.81	4.23	52.3
17	Sodium ion (Na⁺)	cmol/kg	152	0.05	0.54	0.18	0.15	0.01	0.11	1.33	1.30	59.0
18	Potassium ion (K⁺)	cmol/kg	152	0.05	0.83	0.28	0.22	0.02	0.19	0.95	-0.03	67.9
19	Calcium ion (Ca²⁺)	cmol/kg	152	0.09	5.79	1.45	1.09	0.10	1.20	1.28	1.31	82.5
20	Magnesium ion (Mg²⁺)	cmol/kg	152	0.10	4.67	2.14	1.89	0.09	1.15	0.33	-1.12	53.9
21	CEC	cmol/kg	152	0.96	12.14	4.73	4.49	0.20	2.43	0.58	-0.47	51.4
22	Base Saturation	%	152	36.0	95.2	82.3	84.5	0.82	10.06	-1.50	3.17	12.2
Major anions, Nutrient compounds and Organic Matter Contents of Sediment
23	Chloride (Cl⁻)	µgg^− 1	152	3.60	71.00	31.46	28.40	1.05	12.88	0.70	0.54	40.9
24	Sulphate (SO₄²⁻)	µgg^− 1	152	1.27	1053.19	172.56	71.40	18.31	225.74	2.05	3.66	130.8
25	Nitrate (NO₃⁻)	µgg^− 1	152	3544	9746	5354.76	5316	144.02	1821.72	-0.77	1.95	34.02
26	Phosphate (PO₄³⁻⁾	µgg^− 1	152	19.203	1368.85	343.71	285.31	26.69	366.37	1.20	0.95	97.86
27	Phosphorus (P)	µgg^− 1	152	5.19	369.96	97.78	79.68	7.36	90.67	1.14	0.76	24.4
28	Total Nitrogen (TN)	%	152	0.08	0.22	0.13	0.12	0.003	0.03	0.63	-0.34	92.7
29	Organic Matter (OM)	%	152	0.69	14.10	3.20	2.69	0.20	2.52	1.38	2.07	78.7
Min. = Minimum value; Max. = Maximum value; S.E. = Standard Error of Mean;
S.D. = Standard deviation; N = Number of Samples; ORP = Oxidation Reduction Potential;
CEC = Cation Exchange Capacity; CV = Coefficient of Variation

Table 3

Seasonal variation in the hydro-physical and meteorological parameters of Owalla Reservoir
S/N	Parameter	Unit	N		Season
					Dry Season	Rainy Season	ANOVA
					Mean ± S.E.	Mean ± S.E.	F	P
	Hydro-physical and Meteorological Parameters (Overall Seasonal Variations)
1	Depth	M	68		5.28 ± 0.40	5.05 ± 0.41	0.155	0.694
2	Transparency	M	68		1.60 ± 0.05	1.52 ± 0.05	1.080	0.301
3	Air Temperature	^oC	76		31.9 ± 0.29	28.0 ± 0.20	125.7	1.41X10^− 21***
4	Water Temperature	^oC	76		30.6 ± 0.18	29.1 ± 0.10	56.79	4.26 X10¹²***
5	Sediment Temperature	^oC	76		27.6 ± 0.23	27.4 ± 0.14	0.636	0.426
6	Sand	%	76		77.1 ± 1.80	69.1 ± 2.72	6.092	0.015*
7	Silt	%	76		7.9 ± 0.80	10.9 ± 1.30	3.749	0.055
8	Clay	%	76		14.9 ± 1.27	19.9 ± 1.69	5.443	0.021*
9	pH (in H₂O)		76		5.33 ± 0.09	5.21 ± 0.09	1.017	0.315
10	pH (CaCl₂)		76		4.87 ± 0.07	4.74 ± 0.07	1.607	0.207
11	Conductivity	µScm^− 1	76		526.6 ± 61.29	643.3 ± 67.41	1.641	0.202
12	ORP (in H₂O)	mV	76		81.5 ± 5.08	89.0 ± 4.92	1.122	0.291
13	ORP (CaCl₂)	mV	76		107.8 ± 4.17	115.2 ± 3.86	1.687	0.196
14	Aluminum ion (Al³⁺)	cmol/kg	76		0.27 ± 0.02	0.30 ± 0.02	1.183	0.279
15	Hydrogen ion (H⁺)	cmol/kg	76		0.39 ± 0.03	0.41 ± 0.02	0.182	0.670
16	Sodium ion (Na⁺)	cmol/kg	76		0.18 ± 0.01	0.18 ± 0.01	0.175	0.677
17	Potassium ion (K⁺)	cmol/kg	76		0.26 ± 0.02	0.30 ± 0.02	1.416	0.236
18	Calcium ion (Ca²⁺)	cmol/kg	76		1.45 ± 0.15	1.45 ± 0.13	0.0002	0.988
19	Magnesium ion (Mg²⁺)	cmol/kg	76		2.09 ± 0.13	2.18 ± 0.14	0.253	0.616
20	CEC	cmol/kg	76		4.64 ± 0.29	4.82 ± 0.27	0.207	0.650
21	Base Saturation	%	76		82.34 ± 1.19	82.27 ± 1.12	0.002	0.964
22	Chloride (Cl⁻)	µgg^− 1		76	29.38 ± 1.54	33.54 ± 1.38	4.037	0.046*
23	Sulphate (SO₄²⁻)	µgg^− 1		76	165.98 ± 28.93	179.13 ± 22.62	0.128	0.721
24	Nitrate (NO₃⁻)	µgg^− 1		76	5166.49 ± 194.31	5543.04 ± 211.75	1.717	0.192
25	Phosphate (PO₄³⁻)	µgg^− 1		76	312.0 ± 35.46	375.34 ± 39.55	1.419	0.2354
24	Phosphorus (P)	µgg^− 1		76	88.78 ± 9.83	106.78 ± 10.91	1.502	0.222
25	Total Nitrogen (N)	%		76	0.12 ± 0.003	0.13 ± 0.003	3.196	0.076
26	Organic Matter (OM)	%		76	3.27 ± 0.30	3.13 ± 0.28	0.118	0.732

Table 4

Spatial variation (Length) in physico-chemical characteristics of sediments along the major axis of Owalla Reservoir
S/N	Parameter	Unit	Reach			ANOVA
S/N	Parameter	Unit	Upstream Mean ± S.E.	Mid-Basin Mean ± S.E.	Downstream Mean ± S.E.	F	P
Hydro-physical and Meteorological (N = 32) (N = 80) (N = 24)
1	Depth	M	4.05 ± 0.34	4.73 ± 0.24	8.12 ± 1.17	14.270	2.437X10^− 6***
2	Transparency	M	1.36 ± 0.07	1.55 ± 0.04	1.88 ± 0.08	13.430	4.889 X10^− 6***
(N = 32) (N = 90) (N = 30)
3	Air Temperature	⁰C	29.2 ± 0.47	30.5 ± 0.31	29.1 ± 0.52	3.818	0.024*
4	Water Temperature	⁰C	29.6 ± 0.26	30.1 ± 0.15	29.3 ± 0.28	3.717	0.027*
5	Sediment Temperature	⁰C	27.3 ± 0.25	27.6 ± 0.16	27.3 ± 0.38	0.923	0.400
Sediment Physical Characteristics (N = 32) (N = 90) (N = 30)
6	Sand	%	66.9 ± 4.59	72.7 ± 2.05	80.9 ± 2.52	3.826	0.024*
7	Silt	%	12.8 ± 2.15	9.5 ± 0.95	5.5 ± 1.10	4.913	0.009**
8	Clay	%	20.2 ± 2.88	17.7 ± 1.32	13.6 ± 2.04	1.988	0.141
Sediment General Chemical Characteristics (N = 32) (N = 90) (N = 30)
9	pH (in H₂O)		5.09 ± 0.10	5.27 ± 0.08	5.46 ± 0.17	1.883	0.156
10	pH (CaCl₂)		4.62 ± 0.08	4.81 ± 0.06	4.99 ± 0.13	3.206	0.043*
11	Conductivity	µScm^− 1	512.5 ± 74.77	591.7 ± 59.36	642.0 ± 125.92	0.423	0.656
12	ORP (in H₂O)	mV	95.8 ± 5.92	85.1 ± 4.51	74.4 ± 9.69	1.897	0.154
13	ORP (CaCl₂)	mV	123.1 ± 4.39	111.2 ± 3.58	99.7 ± 8.01	3.548	0.031*
Sediment Exchangeable Cations (N = 32) (N = 90) (N = 30)
14	Aluminum ion (Al³⁺)	cmol/kg	0.25 ± 0.02	0.28 ± 0.02	0.32 ± 0.04	1.354	0.261
15	Hydrogen ion (H⁺)	cmol/kg	0.33 ± 0.02	0.40 ± 0.02	0.45 ± 0.05	2.802	0.064
16	Sodium ion (Na⁺)	cmol/kg	0.18 ± 0.02	0.19 ± 0.01	0.16 ± 0.02	0.913	0.404
17	Potassium ion (K⁺)	cmol/kg	0.24 ± 0.03	0.30 ± 0.02	0.27 ± 0.04	1.334	0.266
18	Calcium ion (Ca²⁺)	cmol/kg	1.42 ± 0.18	1.59 ± 0.13	1.09 ± 0.22	2.011	0.138
19	Magnesium ion (Mg²⁺)	cmol/kg	2.37 ± 0.19	2.22 ± 0.13	1.62 ± 0.16	4.093	0.019*
20	CEC	cmol/kg	4.80 ± 0.39	4.98 ± 0.27	3.90 ± 0.42	2.281	0.106
21	Base Saturation	%	85.30 ± 1.34	82.90 ± 1.07	77.35 ± 1.95	5.526	0.005**
Major Anions, Nutrient and Organic Matter (N = 32) (N = 90) (N = 30)
22	Chloride (Cl⁻)	µgg^− 1	29.95 ± 2.20	32.15 ± 1.40	31.00 ± 2.27	0.363	0.696
23	Sulphate (SO₄²⁻)	µgg^− 1	137.69 ± 25.44	179.51 ± 25.02	188.86 ± 47.81	0.499	0.608
24	Nitrate (NO₃⁻)	µgg^− 1	5184.38 ± 240.14	5385.23 ± 229.11	5094.5 ± 247.84	1.695	0.1869
25	Phosphate (PO₄³⁻)	µgg^− 1	408.59 ± 54.72	364.62 ± 40.70	290.36 ± 44.53	1.534	0.2189
26	Phosphorus (P)	µgg^− 1	110.43 ± 14.79	105.58 ± 10.09	60.91 ± 13.47	3.216	0.043*
27	Total Nitrogen (TN)	%	0.13 ± 0.005	0.13 ± 0.003	0.11 ± 0.005	3.165	0.045*
28	Organic Matter (OM)	%	3.52 ± 0.40	3.43 ± 0.28	2.19 ± 0.36	3.113	0.047*
* = Significant difference (p ≤ 0.05)
** = Highly significant difference (p ≤ 0.01)
*** = Very highly significant difference (p ≤ 0.001)

Table 5

Spatial variation (Breadth) in physico-chemical Characteristics of sediments across the major axis of Owalla Reservoir
S/N	Parameter	Unit	Region		ANOVA
S/N	Parameter	Unit	Open-Water Mean ± S.E.	Littoral Mean ± S.E.	F	P
Hydro-physical and Meteorological (N = 56) (N = 80)
1	Depth	M	7.19 ± 0.55	3.75 ± 0.16	47.410	2.025X10^− 10***
2	Transparency	M	1.57 ± 0.06	1.55 ± 0.04	0.029	0.864
(N = 56) (N = 96)
3	Air Temperature	⁰C	29.9 ± 0.42	30.0 ± 0.29	0.001	0.977
4	Water Temperature	⁰C	29.7 ± 0.20	29.9 ± 0.15	0.360	0.549
5	Sediment Temperature	⁰C	26.6 ± 0.19	28.0 ± 0.16	31.780	8.329X10^− 8***
Sediment Physical Characteristics (N = 56) (N = 96)
6	Sand	%	68.8 ± 2.73	75.6 ± 2.06	3.909	0.050*
7	Silt	%	10.6 ± 1.23	8.7 ± 0.98	1.409	0.237
8	Clay	%	20.4 ± 1.85	15.7 ± 1.29	4.489	0.036*
Sediment General Chemical Characteristics (N = 56) (N = 96)
9	pH (in H₂O)		4.75 ± 0.08	5.57 ± 0.07	57.090	3.811X10^− 12***
10	pH (CaCl₂)		4.39 ± 0.06	5.05 ± 0.05	60.080	1.275X10^− 12***
11	Conductivity	µScm^− 1	828.3 ± 91.70	443.1 ± 42.78	18.490	3.068X10^− 5***
12	ORP (in H₂O)	mV	115.1 ± 4.87	67.8 ± 3.85	57.020	3.913X10^− 12***
13	ORP (CaCl₂)	mV	136.0 ± 3.69	97.2 ± 3.16	60.040	1.294X10^− 12***
Sediment Exchangeable Cations (N = 56) (N = 96)
14	Aluminum ion (Al³⁺)	cmol/kg	0.31 ± 0.03	0.26 ± 0.01	2.960	0.087
15	Hydrogen ion (H⁺)	cmol/kg	0.48 ± 0.04	0.35 ± 0.02	14.840	1.730X10^− 4***
16	Sodium ion (Na⁺)	cmol/kg	0.21 ± 0.02	0.16 ± 0.01	7.631	0.006**
17	Potassium ion (K⁺)	cmol/kg	0.35 ± 0.03	0.24 ± 0.02	10.570	0.001***
18	Calcium ion (Ca²⁺)	cmol/kg	1.40 ± 0.16	1.49 ± 0.12	0.188	0.665
19	Magnesium ion (Mg²⁺)	cmol/kg	2.34 ± 0.14	2.02 ± 0.12	2.798	0.096
20	CEC	cmol/kg	5.09 ± 0.31	4.52 ± 0.25	1.904	0.170
21	Base Saturation	%	82.39 ± 1.15	82.26 ± 1.11	0.006	0.940
Major Anions, Nutrient and Organic Matter (N = 56) (N = 96)
22	Chloride (Cl⁻)	µgg^− 1	33.22 ± 1.87	30.43 ± 1.24	1.659	0.200
23	Sulphate (SO₄²⁻)	µgg^− 1	284.92 ± 39.97	107.01 ± 13.47	25.540	1.247X10^− 6***
24	Nitrate (NO3-)	µgg^− 1	6067.52 ± 192.87	4988.37 ± 200.52	12.84	4.59X10^− 4***
25	Phosphate (PO₄³⁻)	µgg^− 1	436.93 ± 49.14	293.51 ± 30.25	6.861	9.67 X10^− 3***
26	Phosphorus (P)	µgg^− 1	118.09 ± 13.28	85.94 ± 8.52	4.551	0.035*
27	Total Nitrogen (TN)	%	0.14 ± 0.004	0.12 ± 0.003	9.156	0.003**
28	Organic Matter (OM)	%	3.79 ± 0.38	2.86 ± 0.23	4.899	0.028*
* = Significant difference (p ≤ 0.05)
** = Highly significant difference (p ≤ 0.01)
*** = Very highly significant difference (p ≤ 0.001)

Seasonal variations in the physico-chemical properties of sediments from Owella reservoir is as presented in Table 3. Higher mean values of depth, transparency, air temperature, water temperature and sediment temperature (5.28 ± 0.40 m; 1.60 ± 0.05 m; 31.90 ± 0.29°C; 30.6 ± 0.18°C and 27.6 ± 0.23°C) were recorded during the dry season while mean value for depth was higher in rainy season. There was a high significant difference (p < 0.001) in air and water temperature between the seasons. The mean values for pH (in H₂O), pH (CaCl₂), base saturation and organic matter (5.23 ± 0.09; 4.87 ± 0.07.82.34 ± 1.19% and 3.27 ± 0.03%) were higher during the dry season of the study period. Aluminium, hydrogen ion, potassium, magnesium, CEC, sulphate, nitrate, phosphate, phosphorus and total nitrogen (0.30 ± 0.02 cmol/kg; 0.41 ± 0.02 cmol/kg; 0.03 ± 0.02 cmol/kg; 2.18 ± 0.14 cmol/kg; 4.82 ± 0.27 cmol/kg; 179.13 ± 22.62µgg^− 1; 5543.04 ± 211.75 µgg^− 1; 375.34 ± 39.55µgg⁻1;106.78 ± 10.91µgg^− 1 and 0.13 ± 0.003%) were higher in the rainy season compared with the values recorded in the dry season. Chloride mean concentration was higher during rainy season (35.50 ± 1.38µgg^− 1) and there was significant different (p < 0.05) between seasons.

Spatial variation in the physico-chemical properties of sediments from Owella reservoir is as showing in Table 4. Maximum mean value of depth and transparency was recorded at downstream (8.12 ± 1.17 m); (1.88 ± 0.08 m) and it is significantly higher than those observed at both upstream and mid-basin of the reservoir. However, there was a high significant difference (p < 0.001) along the horizontal variations. The mean value of sand exhibited higher values at downstream (80.90 ± 2.02%) than the other station and there was significant different (p < 0.05) between the stations. The highest mean concentration for conductivity, aluminium, hydrogen ion and sulphate (642.00 ± 125.92 µScm^− 1; 4.32 ± 0.04 cmol/kg; 0.45 ± 0.05 cmol/kg and 188.86 ± 47.81µgg^− 1) were recorded at downstream station. Mean value recorded for pH (CaCl₂) was significantly higher (p < 0.05) at downstream (4.99 ± 0.13) compare to other stations. A strong significant difference (p < 0.001) among the sampling stations were observed in relation to air temperature and water temperature with highest values recorded in Mid-basin of the reservoir (maximum of 30.50 ± 0.31°C and 30.10 ± 0.15°C). The mean concentration recorded for sodium, potassium, calcium, CEC, nitrate and chloride, respectively (0.19 ± 0.01 cmol/kg; 0.30 ± 0.02 cmol/kg;1.59 ± 0.13 cmol/kg; 4.98 ± 0.27 cmol/kg 5385.23 ± 229.11µgg^− 1and 32.10 ± 0.40µgg^− 1) slightly differs at Mid-basin station compare to other stations. Silt value obtained at upstream station (12.80 ± 2.15%) was higher that the values obtained at mid-basin and downstream stations and there was a strong significant difference (p < 0.001) between the stations. The mean value for ORP (CaCl₂) was slightly higher at upstream (123.10 ± 4.39 mV) and there was significant different (p < 0.05) between the stations. Clay, magnesium and ORP (H₂O) mean values recorded at upstream significantly differ from values obtained at the other stations. The mean concentrations obtained for phosphorus, phosphate and organic matter during this study were higher at the upstream (110.43 ± 14.79µgg^− 1, 408.59µgg^− 1, 408.59 ± 54.72µgg^− 1 and 3.52 ± 0.40%) and there was significant difference along the horizontal axis of the reservoir. There was significantly difference in the value observed for base saturation at upstream (85.30 ± 1.34%) compared with others stations.

Generally, there was a high significant difference (p < 0.001) in the mean value obtained for depth at open-water compare to littoral zone while highest mean concentration for silt, clay, aluminium, magnesium, ECE, base saturation and chloride were recorded at open-water zone. Mean concentration for conductivity was significantly higher at open-water than littoral zone of the reservoir. Maximum mean values for ORP (H₂O) and ORP (CaCl₂) were recorded at open-water zone (115.10 ± 4.87 mV and 136.00 ± 3.69 mV) and there was a high significant difference (p < 0.001) between the zones. Higher mean concentration was recorded for sodium, potassium, chloride, sulphate, nitrate, phosphate, phosphorus, total nitrogen and organic matter in the open-water zone compared with the littoral zone and there was significant difference in the mean concentration between the zones. The mean values obtained for transparency, air temperature, water temperature, sand, calcium was higher at littoral zone but mean values were significantly different for sediment temperature, pH (in H₂O), pH (CaCl₂) and hydrogen ion at littoral zone (Table 5). The major dominant ions in sediment analysed from Owella Reservoir are Magnesium (cations) and Sulphate (anions) but contradict the result from trilinear/piper plot were Calcium and chloride are most dominant from the diagram (Fig. 3). The water is alkaline earth exceed alkalies metals and strong acids exceed weak types (Fig. 4).

In aquatic ecosystems, bottom sediments are crucial because they provide habitat for a variety of macro- and epibenthic species. According to pollution levels, sediments serve as a convenient sink for contaminants where they are concentrated. The health of these species and the entire waterbody could be significantly harmed by contaminated or exposed bottom sediments. Understanding the correlations between sediment-associated chemical concentrations and the incidence of unfavorable biological effects is necessary for evaluating the impact of this risk. Sediment quality standards are scientific instruments that compile knowledge on the connections between chemical concentrations in sediment and any negative biological effects brought on by exposure to these pollutants. The parameter analyses also revealed high values for the CVs, and Han et al. (2006) found that while the CV values of elements impacted by non-natural sources (biological and anthropogenic) are very high, those of elements dominated by natural sources are comparatively low (Han et al. 2006). The pH (in H₂O) and pH (CaCl₂) CV values in the current study were > 50%, which may be related to the fact that these variables have a defined range and that any influence on them will change either their acidity or alkalinity. The concentrations of the other parameters can be affected at any given time by factors including the parameter's residence time, rate of input along with the concentration, interactions between the parameters, and the current physical ambient circumstances. Sand (73.1%), clay (17.4%), and silt (9.4%) are the overall mean percentage values for sediment texture acquired in this study, which are similar to earlier investigations on sediment composition in Nigeria waterbody. Depending on the parent materials and components from the nearby catchment area, the texture may vary.

The dominant amount of sand in the Owella reservoir may be due to the kind of soil geology in the region and input from surface runoffs, which also contribute to an increase in the percentage of sand there. This study compares favorably to the work of George et al. (2010), which found that sand (73.97%), clay (22%) and silt (27%) were present in Okpoka Creek. However, Ezekiel et al. (2010) found slightly different results, reporting that sand (88.6%), clay (7.09%) and silt (4.31%) made up a percentage contribution to their studies on the Sombreiro River. The narrow temperature ranges for air, water, and sediment temperatures obtained during this study period (24.4–36.0°C, 27.0–33.0°C, and 24.0–34.0°C, respectively) show that the tropical zone does not change as much as the temperate zone. As a result, the influence of temperature on the biota and/or the ecosystem in a typical tropical reservoir that is permanently filled with water throughout the year is typically very small or negligible (Ajuzie, 2012). Wantzen et al. (2006) noticed that tropical locations typically suffer some notable degree of seasonality in rainfall rather than substantial thermal seasonality. Overall Mean pH (in H₂O) and pH (in CaCl₂) concentrations in Owalla Reservoir sediment were 5.27 0.06 and 4.89 0.05, respectively, and were above WHO (2017) allowed limits. The large intake of organic matter from nearby habitation, agriculture, and washing activities may be the cause of the pH concentration values observed in this study. The sediments' acidic pH value may cause the Owalla waterbody to become acidic. Additionally, metals that can be hazardous to aquatic life are mobilized by acidic conditions (e.g., aluminium). Through prolonged stress, which damages health and reduces the affected individual's capacity to find food, shelter, or mates for reproduction, metal toxicity can result in reduced survival in fish (Mohan and Kumar, 1998). Nathaniel (2001) in the Opa reservoir and Asibor (2008) in the Asejire Reservoir, both in Nigeria, had reported findings that were comparable. These studies' sediment is typically acidic, which contrasts with the acidic pH ranges of 2.70 ± 0.10 to 5.5 ± 0.31 obtained by Umesi (1999) in Rumueme Creek, Port Harcourt, and 5.06–5.85 reported by Ezekiel et al. (2011) for the Sombreiro River. These studies' sediment is typically acidic, which contrasts with the acidic pH ranges of 2.70 ± 0.10 to 5.50 ± 0.31 obtained by Umesi (1999) in Rumueme Creek, Port Harcourt, and 5.06–5.85 reported by Ezekiel et al. (2011) for the Sombreiro River. The variances in ionic content between the fresh water areas are what produce the significant discrepancy between the conductivity of the sediment in this study and those of those areas. A process known as eutrophication, which essentially results in biological death in the Owalla reservoir due to a lack of bio-available oxygen, may have been accelerated by surface runoffs, which may have increased the level of phosphate and nitrate from household waste deposits into the water, from washing, and from agricultural activities. Different anthropogenic activities carried out in the various water bodies may be the cause of variations in the physico-chemical quality of the sediments acquired from this investigation compared to earlier studies. Typically, human activities and natural factors have a significant impact on the sediment physico-chemical quality of any water body. In the current study, the organic matter content of sediments ranged from 0.69 to 14.10%, with a mean of 3.20 ± 0.20. The range of organic matter indicated that the sediments ranged from being low to being organic in nature, and the average could be considered to have a moderate amount of organic matter. Generally speaking, sediments are considered to be organically rich if their organic matter content is greater than 1%. (Chandrakiran and Kuldeep, 2013).

All of the physico-chemical parameters of bottom sediment investigated in Owalla reservoir are significantly impacted by seasonal fluctuations. Except for the sediment temperature, which was slightly lower in the rainy season, the mean values of air, water, and sediment temperature obtained in this study for the dry season (31.90 ± 0.29°C, 30.60 ± 0.18°C, and 27.60 ± 0.23°C, respectively) and rainy season (28.00 ± 0.20°C, 27.40 ± 0.14°C, and 27.40 ± 0.14°C) are significantly different. During the warmer months of the year, the heat from the water is significantly absorbed by lake sediment, and during the colder months, the heat is transferred to the water (Wetzel, 2001). The greater temperatures observed during the dry season than during the rainy season might likely be ascribed to the dry season's typical low relative humidity and high air temperature. The highest mean values of clay and silt were found during the rainy season, whereas the highest mean values of sand were found during the dry season. These considerable differences in clay and sand values between seasons were observed. Although the average depth during the dry season was higher than what was seen during the rainy season period, this difference was not statistically significant (p > 0.05). In the Ribeirao das Lajes Reservoir in Rio de Janeiro, Brazil, Guarino et al. (2005) reported that the water level (depth) was highest in the rainy season in April and lowest in the dry season in November. The unusually lower water volume (reduced flooding) of the reservoir in the rainy season compared to the dry season along with overland runoff occurring predominantly during the rainy season may be the cause of the much higher mean sand value during the rainy season than during the dry season. According to Ong et al. (2012), the rainy season is characterized by coarser sediments due to excessive rainfall and stronger water currents. The entry of acid hydrides by precipitation may be responsible for the drop in pH during the rainy season of this study. Except for sand, which has a greater mean percent in the dry season, the mean percentages measured for clay and silt were higher during the rainy season and lower during the dry season. This outcome could be attributed to surface runoffs during the rainy season that contain substantial amounts of clay and silt, whereas the higher percentage of sand during the dry season is a result of water concentration and heavy sand sedimentation in the waterbody. Both the dry season of the research (5.33 ± 0.09, 4.87 ± 0.07) and the rainy season (5.21 0.09 and 4.74 0.07) have extremely acidic mean pH concentrations measured in the reservoir. Metal cations that have been adsorbed onto the surface of sediment compete with hydrogen ions at low pH, which causes them to remobilize into the water column. The oxidation of FeS to H₂SO₄ may be the cause of the comparatively low pH value during the wet season (Ramanathan, 1997).

In addition to the impact of fresh water, variations in pH values could be linked to redox changes in sediments and the water column (Holmer et al., 1994). The alkaline characteristics of the water from Owalla reservoir can be related to the presence of carbonates and bicarbonates ions, while the acidic characteristics can be attributed to organic acids from decaying plants (Jain and Agarwal, 2012). Dilution of the water from surface run-offs, which would have raised the level of ions in the waterbody, may be the cause of the river's electrical conductivity, which was higher in the rainy season (643.30 ± 67.41 Scm^− 1) but lower in the wet season (526.60 ± 61.29 Scm^− 1). Aquatic system temporal fluctuations can have both direct and indirect effects on the variables regulating nutrient flows (Thayer, 1971). Seasonal trends have been shown to alter how nutrients are distributed in any waterbody (Baird and Ulanowicz, 1989; Morris, 2000). Seasonal cycles result from imbalances in the mineralization and consumption processes (Morris, 2000). Higher levels of phosphorus (106.78 ± 10.91 gg-1), nitrate (5543.04 ± 211.45 gg-1), and phosphate (375.34 ± 39.55 gg-1) observed during the rainy season may be attributable to dead organic matter from the top layer, while lower levels observed during the dry season may be attributable to the removal of the top layer of sediments by significant flooding during the rainy season. The discharge of household wastes into the river as a result of rainfall, the subsequent sedimentation of suspended particulates from phosphate and nitrogen fertilizers, and these factors may have all played a role in the rise in the phosphorus content of the sediment (Tukura et al., 2005 and Ekeanyanwu et al., 2010). However, discharges containing fertilizers, pesticides, insecticides, and herbicides used by farmers near the water body's catchment area that are washed into the reservoir during the rainy season may be to blame for the phosphate that was abnormally high during the rainy season (Swant et al, 2011). The study's findings about the high levels of phosphorus from run-off during rainy seasons could possibly be explained by the fact that cropland is primarily used around riverine areas in Nigeria. Phosphorus can enter rivers from a number of sources, including solid rock deposits, surface catchment runoff, and interactions between water and silt from dead plant and animal remnants at river bottoms. The nutrient phosphate is regarded as the most important among those responsible for the eutrophication of lakes since it is the main element that spurs the growth of algae and ultimately lowers the levels of dissolved oxygen in the water (Murdoch et al., 2001). The average nitrite concentration across the globe has been calculated to be 1 mg of nitrite/liter, and it is usually believed that nitrite concentrations in freshwaters are minimal (Stanley and Hobbie, 1981; Paul and Clarke, 1989). (Meybeck, 1982).

Because of nutrient enrichment, productivity, degradation, and sedimentation, the watershed's nutrient load is excessive, and as a result, rivers may be destroyed (Adeyemo, 2003). For development, reproduction, and survival of organisms, nitrate, a kind of nitrogen, is an essential nutrient. Nitrate concentrations above 1 mgL^− 1 are harmful to aquatic life (Johnson et al., 2000). Low levels of total nitrogen recorded during the dry season may be attributed to the low level of organic matter along the river, while high levels of total nitrogen may be caused by the oxidation of dead plant organic matter that collected on top layer. A higher value of total nitrogen during the rainy season may be caused by riparian cultivators applying nitrogenous fertilizers, or by more nitrogenous organic and inorganic debris being washed into the reservoir from catchment basins during the time of rainfall (Swant et al, 2011). This is due to the fact that initial rains remove deposited nitrate from near-surface soils and that nitrate levels substantially decrease as the rainy season goes on. By introducing leached and eroded materials into the waterbody, organic materials in river sediment are transported from terrestrial biota (allochthonous sources) and primary production within aquatic ecosystems (autochthonous sources). Due to the high concentration of organic matter during the dry season or the nature of the sediment, as well as the rapid rate of sedimentation and decomposition of foliage and other vegetative remains in the sediment, the organic matter content of the sediment increased to 3.72 ± 0.30%. (Saravanakumar et al., 2008).

The concentration of exchangeable cations (Al³⁺, H⁺, Na⁺, K⁺, Ca²⁺, and Mg²⁺), anions/nutrients compounds (Cl⁻ and SO₄²⁻); P and N) and organic matter generally showed to be significantly higher in rainy season than dry season. According to Swant et al. (2011), the weathering of minerals and eventual deposition of those minerals in sediment during the rainy season may be the cause of the greater concentrations of exchangeable cations and anions during that time. Since sampling at the basin typically took place in the afternoon, when solar radiation and heat absorption are at their highest, the mid-higher basin's significant difference in mean values for air and water temperature over those of other stations is presumably related to this. In comparison to the mid-basin and upstream (lotic) basins, the mean sand fraction of the reservoir sediments was much higher at the downstream (lentic) basin. This might be because most of the stations built downstream of the reservoir were primarily static and located near the littoral zone. In the current study, the pH means values varied amongst the stations (upstream, 5.09 0.10 to 4.62 ± 0.08, mid-basin, 5.27 ± 0.08 to 4.81 ± 0.06, and downstream, 5.46 0.17 to 4.99 013). The pH means values (in H₂O) and pH mean values (in CaCl₂) were also different. It is not surprising that the sediments in this study have acidic pH ranges because anaerobic processes in the bottom sediments are typical of most freshwater bodies in Nigeria (Ogbeibu et al, 2014). Contrary to the findings of Ogbeibu et al. (2014), no uniformity in the grouping of the mean potential of hydrogen ions in respect to either the lotic or the lentic systems was found for the Benin River. The existence of several fish species in the aquatic environment will be threatened by the pH mean concentration range discovered in this inquiry, which lies between the ranges of 5.09 and 5.46 and 4.62 and 4.99. Natural sources of organic matter included plant materials deposited on sediments and human contributions to aquatic systems (Adeyemo et al, 2008). When compared to the findings from the work of Adeyemo et al. (2008) in Ibadan city, the organic matter concentrations obtained in this investigation were significantly higher (values > 0.31–1.14% of the mean range in this study). These amounts above what was previously reported for the sediment of the Benin River (Ogbeigu et al. 2104). Extreme organic matter concentrations, according to Hyland et al. (2000), may harm benthic communities. Since the downstream can operate as a sink for several ion species as well as other byproducts of heavy human activity at the station, electrical conductivity has higher mean concentrations there than at other stations. The concentrations were discovered to be comparable to those observed in the Warri River (Hyland et al. 2000), but significantly lower than those observed in the Benin River (Ogbeibu et al. 2014) and the Adoni flats in the Niger Delta (Ansa and Francis) (Olomukoro and Egborge, 2004). These variations can be related to how these various aquatic environments and salt water within the ecological type are used for activities at the shed. The sediment's nutrient contents were very similar to those recorded for the country's main rivers. The average phosphorus concentrations appear to be higher upstream and lower downstream. Regarding the spatial patterns in the reservoir regime, there was no discernible fluctuation in the mean concentration of chemical properties such sulphate, chloride, and total nitrogen. The values found in this study fell within the range reported by Kolo et al. (2010) for the Lake Chad Basin in Borno State, Nigeria, but were greater than those of the Warri River (Ogbeibu et al., 2014). These illustrate the nutrients' non-point source input and the strong impact of biological and artificial activities on the nutrients. The lentic habitats' sodium and potassium concentrations varied noticeably; this might be explained by the lentic environments' long water retention times and sink-like characteristics. Alkali metals have a higher possibility of combining with other elements or substances for which they have an affinity because of the water retention time, which was typically longer in the lentic environment than the lotic environment. This could be explained by the fact that both elements have properties that are similar to those of elements found in the earth's crust (both physically and chemically), or by the possibility that these two elements came from the same location in these wetlands, perhaps as a result of weathering processes.

Similar to the trends in the alkali metals, the alkaline earth metals (calcium and magnesium) showed some homogeneity in mean concentrations across all sites. In the upstream and mid-basin, the mean concentrations of calcium and magnesium cations were reportedly greater. The coastal zone of the reservoir had a mean sediment temperature that was substantially higher (p 0.05) than the open water zone. This finding could be explained by Wetzel's (2001) observations that the type of adjacent land forms in the basin appear to be a significant factor in the spatial variation in temperatures within the sediments at similar water depths, as shoreline (littoral) sediments of deep lakes have been found to be significantly warmer than sediment. In the littoral zone compared to the open water zone, the mean value of the sand fraction in the textural structure of the Owalla reservoir was greater. This may be caused by high levels of siltation (sand and gravel) entering water bodies via surface runoff, which tend to settle quickly in shallow water areas whereas finer particles (silt and clay) tend to remain in suspension until they reach the more tranquil deeper water (Mishra, 1980). The distribution of grain sizes is determined by depth, according to a report by Goher et al. (2014); when clay and silt sizes rose with depth, sand fraction fell in the same pattern, albeit some samples may diverge from this trend. The sediment pH levels revealed that the open water zone was more acidic than the littoral zone (significantly lower pH means concentration), and conductivity and oxidation-reduction potential (ORP) mean values were significantly higher in the open water zone than the littoral zone, demonstrating an inverse trend with pH levels. The decomposition of organic matter releases organic acids into the sediments, which in turn causes the pH values of lakes to fall (Wondim and Mosa, 2015). Agoston-Szabo and Dinka had previously demonstrated the substantial inverse association between the pH and ORP mean values of the sediment in the Owella reservoir (2009). The fact that the majority of the exchangeable cations, anions, nutrients, and organic matter contents were higher in that region (i.e., open water) than in the littoral region may be the cause of the higher conductivity value in the open water. The phenomena may be caused by the finer sediment fractions (clay and silt), which tend to accumulate more in the open water region than the littoral region with coarser sediment fractions and are typically found to have high conductivity, CEC values, and richer in organic matter.

Higher conductivity is a sign of richer, more charged ionic species, whereas lower electrical conductance is a sign of high silicate materials in sediments. Conductivity or salinity depends on the percentage of ions such as chloride, sulfate, phosphate, bicarbonate, sodium, potassium, calcium, magnesium, etc (Swant et al, 2011). In comparison to the littoral section of the reservoir, the concentrations of nearly all exchangeable cations, anions/nutrient compounds, and organic matter in the sediment of the Owella reservoir revealed significantly higher mean values in the open water region. This can be explained by the fact that the reservoir's open water zone's mean clay concentration of the sediments was much higher than the littoral region. According to Fonseca et al., 2003, finding the fraction of exchangeable cations can change when particle size changes and vice versa depending on the adsorption and cationic exchange processes. In general, the sample points in the piper diagram were grouped into 6 areas based on the Trilinear/or piper plot diagram that was constructed for the Preambular area utilizing the analytical data received from the physcio-chemical analysis: There are six different types: Mg²⁺-NO₃⁻-PO₄³ type; (2) Na⁺-Cl⁻ type; (3) Ca²⁺-Mg²⁺-Cl⁻ type; (4) Ca²⁺-Na⁺- NO₃⁻PO₄³ type; (5) Ca²⁺-Cl⁻ type and (6) Na-NO₃⁻PO₄³⁻ type. Sediment in the current investigation, however, matched Ca²⁺-Cl⁻ kinds. Trilinear or Piper plot analysis of the sediment types indicates that the contribution from the weathering of pyroxenes and amphibole in the hard rocks is clearly discernible. Sulfate was the most prevalent cation and magnesium was the most prevalent anion in the sediment samples taken from the Owalla reservoir. This may be because of the area's high terrain, which suggests an inverse ion exchange process. Rock weathering causes calcium and magnesium to enter the water body throughout this process. Sediments in the lower topographic area, however, are primarily composed of magnesium and sulfate ions. Alkaline Earth outperformed alkali metals in this study's sediment type, while strong acid anions outperformed weak acid anions (Chadha, 1999). The six fields as mentioned by Chadha (1999) is given in below. (1) Alkaline earths exceed alkali metals; (2) Alkali metals exceed alkaline earths; (3) Weak acidic anions exceed strong acidic anions (4) Strong acidic anions exceed weak acidic anions (5) Alkaline earths and weak acidic anions exceed both alkali metals and strong acidic anions, respectively.

Based on the results of the present study, it could be inferred that Owalla Reservoir is clean and not contaminated with most of the investigated physico-chemical characteristics of the sediment but rising in the nutrient level observed in this study is alarming, thus, periodic monitoring and preventative measures are required to save the aquatic system from eutrophication. The findings also have important implications for the development of effective potable water management and strategies for the control of point and diffuse-source pollution within the waterbody.

Authors Contribution

Aduwo, I. A. – writing (Original draft manuscript), Conception or design of the work, Data collection, and Final approval of the version to be published.

Adesakin, T. A., and Oyewale, A. T. – Writing (review and editing), Data analysis and interpretation, Drafting the article

Adeniyi, I. F. – visualization, Critical revision of the article

Interest statement

I solemnly declare the information mentioned herein is true and correct to the best of my beliefs. All the details provided above are genuine to the best of my belief and knowledge. I hereby declare that the above particulars of facts and information stated are correct to the best of my belief and knowledge.

Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Funding Statement

This research received no external funding and the authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Assessment the effects of Anthropogenic activities on sediment physico-chemical characteristics of Owalla Reservoir, Southwest, Nigeria

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