The analysis results are listed in Table: 1 is the study area before and after the 2016 and 2017 monsoons. Table 1 summarizes the chemical analysis of groundwater and the percentage of Indian standards. The table lists the water quality parameters of 105 groundwater samples (BIS, 2005 and WHO, 2011).
Table: 1. Water quality variables represented by BIS (2005) and WHO (2011) for drinking uses and the number of samples from the Krishna Delta Godavari delta surpassing the maximum permissible values
Chemical Parameters
|
WHO Standard
|
BIS Standard
|
No. of sample Exceed Permissible limit
|
Percentage of sample Exceeding Permissible limit
|
No. of sample Exceed Permissible limit
|
Percentage of sample Exceeding Permissible limit
|
|
|
Pre-Monsoon (2016)
|
Post-Monsoon (2017)
|
pH
|
6.5- 8.5
|
6.5- 8.5
|
2
|
1.9
|
0
|
0.0
|
EC (µS/cm)
|
1500
|
1500
|
73
|
70.2
|
70
|
67.3
|
HCO3- (mg/l)
|
600
|
600
|
25
|
24.0
|
76
|
73.1
|
Cl- (mg/l)
|
600
|
1000
|
32
|
30.8
|
27
|
26.0
|
SO42- (mg/l)
|
400
|
400
|
0
|
0.0
|
0
|
0.0
|
Ca2+ (mg/l)
|
200
|
200
|
8
|
7.7
|
9
|
8.7
|
Mg2+ (mg/l)
|
150
|
100
|
26
|
25.0
|
24
|
23.1
|
Na+ (mg/l)
|
200
|
200
|
73
|
70.2
|
69
|
66.3
|
K+ (mg/l)
|
30
|
12
|
79
|
76.0
|
78
|
75.0
|
TDS (mg/l)
|
1500
|
2000
|
40
|
38.5
|
45
|
43.3
|
pH is a measure of the balance between the concentration of hydrogen ions and hydroxide ions in water. Before the monsoon season, the pH varied between 6.9 and 8.8 (average 8.0) (Fig. 2). Most samples are alkaline. After the monsoon, the pH (Fig. 3) varied between 6.8 and 8.0, with an average value of 7.3. It was observed that all samples were within the allowable range. When Inverse Distance Weighting (IDW) interpolation technology was used to generate a pH map, it was found that the village of Pandurangapuram in Bapatla Mandal had the highest pH of 8.8. According to the BIS standard, the water exceeded the allowable limit. Total Dissolved Solids (TDS) is an important parameter in drinking water and other water quality standards. TDS stands for various minerals in dissolved form in water. In natural water, the dissolved solids are mainly composed of carbonate, bicarbonate, chloride, sulfate, phosphate, silica, calcium, magnesium, sodium, and potassium. The TDS is calculated by Eq. (1).
According to WHO (1993) specifications, the maximum allowable maximum TDS is 500 mg/l and the maximum allowable is 2000 mg/l. In the study area, the pre-monsoon TDS value varied between a minimum of 256 mg/l and a maximum of 18560 mg/l, which indicated that most groundwater samples exceeded the maximum allowable limit. According to Davis and De Wiest (1966), TDS-based groundwater classification (Table. 2) was 29% before the monsoon and 24% was allowed to drink after the monsoon. In the post-monsoon season, 36% is suitable for irrigation before the monsoon, and 39% is suitable for irrigation after the monsoon. As shown in Fig. 4 and Fig. 5, in both seasons, about 30% and 28% of the samples are not suitable for drinking and irrigation, before the monsoon, 5% of the samples are suitable for drinking, and not 9% after the monsoon
Table 2
David and Dewiest classification of groundwater based on TDS (mg/l).
TDS (mg/l)
|
Classification
|
Pre-monsoon (2016)
|
Post-monsoon (2017)
|
Number of samples
|
Percentage (%)
|
Number of samples
|
Percentage (%)
|
< 500
|
Desirable for Drinking
|
5
|
5
|
9
|
9
|
500–1,000
|
Permissible for Drinking
|
30
|
29
|
25
|
24
|
1,000–3,000
|
Useful for irrigation
|
38
|
36
|
41
|
39
|
> 3000
|
Unfit for drinking and irrigation
|
32
|
30
|
29
|
28
|
According to Freeze and Cherry (Table.3), TDS-based groundwater classification analysis of freshwater before and after the monsoon is 33–36% freshwater, and brackish water before and after the monsoon is 63–59% of the season. The high TDS is due to the presence of large amounts of sodium and chloride ions. The high concentration of TDS in groundwater samples is due to the leaching of salt from the soil, and domestic sewage may also penetrate the groundwater
Table: 3 Freeze and Cherry classification of Groundwater based on TDS (mg/l)
|
Pre-monsoon (2016)
|
Post-monsoon (2017)
|
TDS (mg/l)
|
Water Type
|
Number of samples
|
Percentage
|
Number of Samples
|
Percentage
|
<1,000
|
Fresh
|
35
|
33
|
38
|
36
|
1,000 - 10,000
|
Brackish
|
66
|
63
|
62
|
59
|
10,000 -1,00,000
|
Saline
|
4
|
4
|
5
|
5
|
>1,00,000
|
Brine
|
0
|
0
|
0
|
0
|
Total Hardness (TH) depends on the calcium and magnesium content of water is calculated by the flowing Eq. (2). The maximum and minimum value of TH in the water samples is 140 mg/l to 15935 mg/l with an average of 1258 mg/l in the pre-monsoon and 99 mg/l to 19969 mg/ in the post-monsoon season. As per the WHO standards, the desirable limit of TH is 100 mg/l whereas the maximum permissible level is 300 mg/l.
TH was above the maximum permissible limit of 300 mg/l in 87 groundwater samples are considered as very hard type water (Table. 4), 16 % of the groundwater samples fell under hard type water in pre-monsoon season and 83% of groundwater has very hard type water in post-monsoon, as well as 1% and 20% are moderately high, hard type in post-monsoon season shown in fig: 6 and fig: 7
Table: 4 Classification of groundwater based on TH (mg/l)
|
Pre-monsoon (2016)
|
Post-monsoon (2017)
|
Total Hardness (mg/l)
|
Type of water
|
Number of samples
|
Percentage
(%)
|
Number of samples
|
Percentage
(%)
|
<75
|
Soft
|
0
|
0.0
|
0
|
0.0
|
75 - 150
|
Moderately high
|
1
|
1.0
|
1
|
1.0
|
150 - 300
|
Hard
|
17
|
16.2
|
20
|
19.2
|
>300
|
Very Hard
|
87
|
82.9
|
83
|
79.8
|
The Electrical Conductivity (EC) value ranges from 400 µS/cm to 29000 µS/cm (pre-monsoon) and 400 µS/cm to 45000 µS/cm (post-monsoon) with an average of 4018 µS/cm and 4451 µS/cm during pre-monsoon and,post-monsoon season respectively is shown in Fig. 8 and Fig. 9. The high EC values in the Krishna area appear pre and post the monsoon. It is the measurement of all soluble salts in the sample, which is the most important water quality standard for crop productivity, that is, water and salt damage. The main effect of high EC water on crop productivity is that plants cannot compete with ions in soil solutions for water. Even though the soil may show wet, because plants can only transpire pure water, useable plant water in the soil solution decreases significantly as EC increases. The amount of water transpired through a crop was directly related to yield. Therefore, irrigation water with high EC reduces yield potential. The present study indicated that overall the water quality was medium to high in the EC category.
The Carbonate (CO3) value is a maximum of 48 mg/l in pre-monsoon with an average of 6.5 mg/l. The Bicarbonate (HCO3) value ranges from 73 mg/l to 1159 mg/l in pre-monsoon and 97 mg/l to 2293 mg/l during the post-monsoon period with an average of 458 mg/l and 947 mg/l. The high HCO3 values towards the North-East direction are seen in the post-monsoon period whereas isolated distribution is seen in the pre-monsoon season. The Chloride (Cl−) value ranges from 56.7 mg/l to 8820 mg/l in pre-monsoon and 71.0 mg/l to 11928.0 mg/l during the post-monsoon period with an average of 947.5 and 957.6 mg/l during the pre-monsoon and post-monsoon respectively. The high chloride values are shown in Nizampatnam Mandal in a post-monsoon period whereas a northward trend is seen in pre-monsoon (Fig. 10 and Fig. 11)
The Calcium (Ca2+) value ranges from 20 mg/l to 528 mg/l in pre-monsoon and 32 mg/l to 464 mg/l during post-monsoon period with an average of 99.4 mg/l and 122 mg/l during pre-monsoon and post-monsoon respectively. The high calcium values are seen near coastal areas towards the stream mouth area in both pre-monsoon and post-monsoon periods. The Magnesium (Mg2+) value ranges from 4.8 mg/l to 651 mg/l in pre-monsoon, and 4.8 mg/l to 849.6 mg/l during the post-monsoon period with an average of 80.6 and 98.2 mg/l during pre-monsoon and post-monsoon respectively. The post-monsoon period high Magnesium values are seen in Machilipatnam Mandal, Sakthinethipalle river mouth area, Uppalaguptam, and Nizampatnam near the coastal region and the north parts of the Godavari area where low magnesium occur.
The Sulphate (SO4) value ranges from 2.4 mg/l to 224.8 mg/l (pre-monsoon) and 2 mg/l to 43.7 mg/l (post-monsoon) with an average of 62.3 and 10.3 mg/l. The low SO4 values are seen entire study area in the post-monsoon period, whereas this trend is seen isolated in pre-monsoon. The Sodium (Na+) value ranges from 40.8 mg/l to 5006.8 mg/l (pre-monsoon) and 9.3 mg/l to 8285 mg/l (post-monsoon) with an average of 634 mg/l and 670.8 mg/l. The Potassium (K+) value ranges from 1.6 mg/l to 443 mg/l (pre-monsoon) and 1.9 mg/l to 712.2 mg/l (post-monsoon) with an average of 60.4 mg/l and 83.1 mg/l during pre-monsoon and post-monsoon respectively. In pre-monsoon high potassium values are seen in Machilipatnam mandal and it is slightly varying in post-monsoon season.
5.1 Piper Trilinear Diagram:
The concentration of major anions and cations can be plotted on the Piper Trilinear graph to understand the geochemical evolution of groundwater. Use AquaChem 2014 software to transpose different groundwater chemical evolution paths and freshwater composition fields (Fig. 12) onto Piper diagrams. Piper diagram is a combination of anion and cation triangles on a common baseline. The diamonds between them are used to characterize different types of water.
The graphical illustration of the ion signature helps to reveal the main ions that control water chemistry. Piper divides water into four types by placing water near the four corners of the diamond. The Water plotted at the top of the diamond is considered as high with Ca2++Mg2+ and Cl−+SO4 − 2, which is the area of permanent hardness. The water plot near the right side corner is rich in Ca2++Mg2+ this water region is temporary hardness. The water plot at the lower corner is composed of alkali carbonates (Na++K+ and HCO3-+CO3 − 2). The water near the left-hand side may be Saline water (Na++ K+ and Cl− + SO4 − 2). It has been observed in Piper's Diagram that the nature of groundwater exists in the Na+- Cl− type study area. Therefore, the Piper diagram can not only identify the nature of water samples but also reveal the relationship between water samples. It is possible to predict and classify geological units and chemically similar water, and then to analyze the trend and flow path of water chemical analysis. In pre-monsoon piper diagram can not only identify the nature of water samples but also reveal the relationship between water samples. It is possible to predict and classify geological units and chemically similar water, and then to analyze the trend and flow path of water chemical analysis. The diagram shows whether the salinity is derived solely from mixing with seawater.