3.1 Body burden of pollutants in snails
Out of the 429 compounds targeted, 30 compounds including PaBs, PPCPs and industrial compounds were detected in snail tissues (Figure 1). Concentrations detected in snail tissues on wet weight basis (ng/g ww) are given in the SI (Table SI-5 and Figure SI-1).
Figure 1: Frequency of detection for compounds in snail tissue samples from the Lake Victoria South Basin. TCEP: Tris(2-chloroethyl)phosphate; NBBS: N-Butylbenzenesulfonamide; NETS: N-Ethyl-o-toluenesulfonamide; BTSA: 2-Benzothiazolesulfonic acid.
Almost two thirds of the chemicals detected in snails were PaBs with 19 out of 30 compounds (Figure 1). Atrazine, a pre-emergence and post-emergence herbicide still in use in Kenya for the control of broadleaf weeds and grasses, was detected in 95% of the snail samples followed by the insect repellant diethyltoluamide (DEET) with 93%. The frequent detection of DEET may be explained by the high mosquito infestation and the high prevalence of malaria infecting 20 to 40% of the population in counties within the lake endemic region of western Kenya (Bashir et al., 2019). DEET is used as a topical insect repellant for the control of mosquitoes. It could be released into the water when performing water-related activities such as taking a bath in the rivers and reservoirs (Thomas et al. 2014), a common practice which was observed during sampling in the region. Individual pesticide concentrations ranged from 0.2 ng/g ww to 375 ng/g ww with maximum concentrations found for the herbicide atrazine at 375 ng/g ww, the fungicide bupirimate at 97 ng/g ww and the insecticide diazinon (37 ng/g ww). The neonicotinoids, acetamiprid and imidacloprid were present in the snail tissues in concentrations up to 27 ng/g ww and 21 ng/g ww, respectively.
Among PPCPs, temazepam (98%) and 2-hydroxyquinoline (78%) had the highest detection frequencies (Figure 1). Individual compound concentrations ranged from 0.8 ng/g ww to 137 ng/g ww. Maximum concentrations were recorded for efavirenz (137 ng/g ww) and 2-hydroxyquinoline (115 ng/g ww). This highest concentration for efavirenz was found in PS28 (Figure SI-1) located within Kisumu County and could be attributed to the high HIV/AIDS prevalence (16.3%) and the access to antiretroviral therapy (ART) with a coverage of 90% (National AIDS Control Council NACC 2018) in the region. Efavirenz is a non-nucleoside reverse transcriptase inhibitor (NNRTI) used in combination with other medications as antiretroviral treatment for HIV/AIDS (Kreitchmann et al. 2019).
Plasticizers and flame retardants were also present in snail tissues with highest detection frequencies reported for tris(2-chloroethyl)phosphate (100%) and N-butylbenzenesulfonamide (NBS, 80%). Individual compound concentrations ranged from 2.8 ng/g ww to 481 ng/g ww. The highest concentration was found for N-ethyl-o-toluene sulfonamide (NETS, 481 ng g-1 ww), a compound with wide-spread use in industrial products and as an ingredient to pesticide formulations. In addition, cotinine was present in all the snails sampled (detection frequency 100%) with concentrations up to 311 ng/g ww (Figure SI-1). Due to the compound being the most abundant metabolite of nicotine, its high stability and half-life, cotinine has been suggested to be the ideal biomarker of tobacco exposure and smoking status (Petersen et al. 2010). Cotinine is excreted by human mainly through the urine and may end up in surface water through effluent discharge of domestic waste.
Variability in compound concentrations was observed in the different snail species sampled from the same site. Example sites include PS17 and PS28 (Figure SI-1) where total concentrations in Ceratophallus sp. and Melanoides tuberculata were lower than those in other species. A plausible explanation for this observation may be different lipid contents and age which could not be considered during sample preparation. A study carried out by Duncan et al. (1987) noted considerable variation in total lipid for Biomphalaria glabrata (5%) and B. alexandrina (2%) snails and, in addition, intraspecific differences in total lipids (1-10%) in B. glabrata. Since most pollutants accumulate primarily in lipid tissue, variation in lipid content may lead to differences in the body burden of contaminants.
The predators of the snails investigated in this study include cray fish (Procambarus alleni), water bug (Belostoma flumineum), waterfowl and cichlid fishes (Monde et al. 2016; Halstead et al. 2018). These predators could biomagnify and bioaccumulate certain contaminants during feeding resulting in elevated levels of contaminants in the food chain. The consumption of contaminated food is a major source for xenobiotics in predating birds and mammals (Nendza et al. 1997). Effects of pesticide exposure in birds have been linked to neurotoxicity and endocrine disruption (Köhler and Triebskorn 2013). Other effects of pesticides include impaired foraging and chick rearing, eggs shell thinning and reproductive failure (Köhler and Triebskorn 2013). In a study carried out by Guo et al. (2016) on compound prioritization based on potential of secondary poisoning in fish-eating birds and mammals, diazepam was ranked the highest (risk score 0.1–1) among the pharmaceuticals tested. In addition, gregarious animals including fish and birds could biomagnify and bioaccumulate contaminants then migrate becoming a predominant pathway for contaminants in the environment (Blais et al. 2007).
3.2 Occurrence and distribution of pollutants in sediments
Out of the 429 targeted compounds analyzed in the sediment samples from the LVSB, 78 compounds were detected (Figure 2 and SI Table SI-6).
Figure 2: Frequency of detection for compounds in sediment samples from the Lake Victoria South Basin. 3,4,5-TCP: 3,4,5-trichlorophenol; 2-MBT: 2-morpholinothiobenzothiazole; 2_IPT: 2-Isopropylthioxanthone; 6:2-FTSA: 6:2 fluorotelomer sulfonic acid
Similar to snail samples, PaBs were the dominant chemical class with 71% of compounds detected in the sediments. Compounds frequently detected included DEET (98%), triclocarban (71%), diuron (65%), pirimiphos methyl (58%) and diazinon (56%) (Figure 2). High detection frequencies of diazinon and pirimiphos-methyl are in line with a study performed by Musa et al. (2011) who noted that diazinon and pirimiphos-methyl were among the commonly used pesticides in the Nyando catchment area which is within the LVSB. Individual compound concentrations reached up to 111 ng/g organic carbon (OC) with highest concentrations recorded for pirimiphos-methyl (111 ng/g OC), diuron (93 ng/g OC) and DEET (68 ng g-1 OC) (Figure 3). Pirimiphos-methyl is a broad spectrum insecticide used for the control of pest during storage. In addition, it is approved as an insecticide for indoor spraying against mosquitoes, cockroaches and houseflies (Pest Control Products Board 2018). Diuron is a selective herbicide for the control of weeds in sugarcane plantations. Concentrations reported in this study are within the range reported in a review by K’oreje et al. (2020) on the occurrence of pesticides in African river sediments.
Figure 3: Compound concentrations and spatial distribution in sediment samples
A total of 19 PPCPs were detected with frequencies ranging from 6% (bezafibrate) to 56% (efavirenz). We also found the preservative propylparaben (33%) and the anti-cancer drug anastrozole (8%). To the best of our knowledge, this is the first study to report anastrozole occurrence in Kenyan aquatic ecosystems. The detection of anastrozole could be linked to the rising diagnoses and treatment of breast cancer in the continent and particularly in Kenya (Ekpe et al., 2019). Anastrozole is applied for hormone therapy during breast cancer treatment. Highest PPCP concentrations were detected for efavirenz with up to 29 ng/g OC. Other compounds detected at higher concentrations include crotamiton (1.8 ng/g OC) and diazepam (1.5 ng/g OC). The concentrations of the antibiotic sulfamethoxazole measured in this study (0.21 ng/g OC) fall below the concentrations reported by Kairigo et al. (2020) in sediments from Mwania river in Kenya by about one order of magnitude. This is probably due to differences in consumption patterns and the impact of municipal waste in the study area since their study was performed in an urban setting.
3.3 Comparison of the incidence of CECs in different environmental matrices of western Kenya
The chemical data from the present study were compared with the compounds found in the water phase in Kandie et al. (2020). In total, 142 compounds were detected in the study area with 79 compounds in water (Figure SI-2), 30 compounds in snails and 78 compounds in sediments. Among these compounds, only nine were common in all three matrices (Figure 4) including acetamiprid, atrazine, azoxystrobin, DEET, diazinon, diuron, imidacloprid, pirimiphos-methyl and triclocarban. Although these compounds were present in all matrices, their ranking with respect to detection frequency and concentrations in the individual matrices was quite different. For example, atrazine and diazinon were among the compounds frequently detected at high concentrations in biota, whereas this was not the case in water and sediments. As expected, rather hydrophobic compounds such as pirimiphos-methyl (log Kow 4.12) were found in higher concentrations in sediments and snails (PS17) than in water.
Figure 4: Numbers of detected compounds in water, sediment and snail samples within the study area
Correlations between concentrations in water, snails and sediments have been found only for few compounds. The strongest correlation was obtained for pirimiphos-methyl (r = 0.53) for concentrations in sediments and snails followed by water and snail concentrations of DEET (r = 0.35) and diazinon (r = 0.31) in sediment and water. The low correlations observed between biota and the other matrices suggest a quite complex bioaccumulation regime involving different uptake pathways (Contardo-Jara et al. 2011), complex temporal exposure patterns (e.g. due to pesticide peaks) and high small-scale variance of exposure.
Among the compounds present, 54 compounds in sediments, 51 in water and 7 in snails were specific to the individual phases (Figure 4), indicating the need to consider different complementary matrices in order to get a more comprehensive picture of contamination. As an example, the pharmaceutical efavirenz and fungicides difenoconazole, bupirimate, flusilazole and tebuconazole were not detected in grab water samples but could be quantified in snail and sediment samples. Efavirenz is moderately hydrophobic (log Kow = 4.7) and is likely to partition to snail and sediment phases. Also, the snapshot character of water samples or the need of enrichment in order to exceed detection limits could influence compound detections. In addition, the intermittent release of chemicals into the environment through a runoff event after spraying or emission events could influence the presence of a compound in the aquatic environment.
3.4 Impact of land use on contamination patterns in different environmental compartments
Among the 48 sampling sites, 17 sites could be clearly connected to specific land uses (Figure SI-3), including agricultural areas (i.e., sugarcane, tea and rice) and reservoirs characterized by low anthropogenic inputs. The other sites showed mixed land use patterns and were not taken into consideration. Overall, sugarcane growing areas showed the highest concentrations of PaBs of all investigated types of land use (Figure 3, Figures SI-1, SI-2 and SI-3).
Sites located in areas without evident anthropogenic influence (PS21, PS39, PS46, PS47 and PS58) were generally less contaminated although total CEC concentrations (1 µg/L) in water from PS 39 indicates hidden wastewater impact as a source for the pharmaceuticals acetyl-sulfamethoxazole (A-SMX) and diphenhydramine, and the industrial compound triethycitrate (Figure 3, Figures SI-1, SI-2 and SI-3). Total CEC concentrations were up to 2 ng/g OC (mainly triclocarban and DEET) and 228 ng/g ww in sediments and snails, respectively. Cotinine, efavirenz, atrazine, N-ethyl-o-toluene sulfonamide and N-butylbenzenesulfonamide were the compounds contributing to the pollution found in snails. Low contaminant concentrations were observed in sites such as in PS46 located within Homabay County with no immediate impact from anthropogenic activities. In addition, a lot of the farmers in this region practice subsistence farming for household consumption, therefore less pesticide inputs may be used in the agricultural practices. The site PS 47 is located closely downstream of the Ruma national park, a wildlife reserve with limited human activities.
Agricultural sites such as PS17 and PS18 within sugarcane plantations had high contaminant concentrations in all the three matrices (Figure 3, Figures SI-1, SI-2 and SI-3). Total compound concentrations were up to 11 µg/L in water, 555 ng/g ww in snails, 304 ng/g OC in sediments. Notably, high concentrations were obtained for the PaBs atrazine in snails and pirimiphos-methyl, DEET and triclocarban in sediments, while 2,4-dichlorophenoxyacetic acid (2,4-D) and hexazinone predominated in water (Kandie et al. 2020). The high concentrations of pesticides are in agreement with sampling during the spraying period in sugarcane plantations (September-October). Other compounds that contributed to pollution included efavirenz and NETs (in both snails and sediments) while the hydrophilic CECs A-SMX, the sweetener acesulfame and triethycitrate were found only in water samples from these two sites. The presence of Awendo town nearby could contribute as an important source of these compounds into the aquatic system.
In rice plantation fields (PS25, PS26, PS27, PS28 and Uli15), high total compound concentrations of up to 3.3 µg/L in water, 598 ng/g ww in snails, 22 ng/g OC in sediments were detected. Similar to sugarcane sites, PaBs contributed most to the overall pollution with atrazine predominating in snails and triclocarban in sediments. Carbendazim and bendiocarb contributed most to water contamination (Kandie et al. 2020).
Among agricultural areas, sites within tea plantations (PS56, PS57, Uli1, Uli3) were least impacted with pollutants. Total CEC concentrations reached up to 6 ng/g OC in sediments with the insect repellant DEET as major contributor, while no snails were found at these sites. A plausible explanation for the low pesticide contamination is the mismatch of sampling (September and October) and the spraying period in tea plantations (June). Additionally, the streams in tea growing areas are protected by wide buffer zones while in other agricultural areas the farmland is much closer to the waterbodies leading to higher run-off potential.
In general, the occurrence of PPCPS in agricultural sites shows evidences of municipal waste impact on the local water bodies. For example, the pharmaceuticals efavirenz and 2-hydroxyquinoline were found in high concentrations in snails collected from rice fields. A plausible explanation for the high concentrations found could be the direct discharge of untreated domestic wastewater from residential areas, lack of sanitation facilities and effluent discharge from wastewater treatment facilities into the river.
3.5 Risk assessment based on sediment concentrations
Based on equilibrium water concentrations calculated from sediment, toxic risks were estimated for fish, crustacean and algae. For fish, the TUsum ranged from 9.1x10-8 to 1.4x10-2 (Table SI-7, Figure SI-4). Pirimiphos-methyl and imidacloprid-urea were identified as risk drivers for fish exposed to sediments in PS17, while diuron is predominating in PS59. Ethyl azinphos contributed greatly to the risk in PS42 and PS43. Maximum TUs were observed for pirimiphos-methyl (TU = 0.007) and the transformation product imidacloprid urea (TU = 0.005), but did not exceed the acute and chronic risk thresholds. However, with this low toxic risk on fish observed, it should be mentioned that natural and synthetic estrogenic hormones were not measured in this study. These hormones have been shown to drive effects on fish reproduction in the ng/L range resulting in the collapse of whole fish populations (Kidd et al. 2007). Estrogens are often emitted with untreated wastewater (König et al. 2017).
Cumulative TUs for crustaceans were generally higher than those obtained for fish (range: 9.6X10-8 to 1.1) with diazinon, fipronil sulfone and pirimiphos-methyl driving the overall risk (Table SI-8, Figure 5). Maximum TUsum was reported for the site PS17 (TUsum 1.1) impacted by the large agro-industrial sugarcane plantation with pirimiphos-methyl (TU = 0.99) and diazinon (TU = 0.11) driving the risk, both exceeding the ART (TU > 0.1) for crustaceans. The chronic risk threshold of TU > 0.001 was exceeded at 20 sites for diazinon, at 14 sites for pirimiphos-methyl and at four sites for fipronil sulfone.
Figure 5: Distribution of risk of toxicity for crustaceans from compounds present in sediment extract based on equilibrium water concentrations. 345-TCP:3,4,5 trichlorophenol
For algae, the cumulative risk was higher than for fish but lower compared to crustaceans. The TUsum ranged from 8.7X10-7 to 0.24, with the photosynthesis inhibitor diuron driving the risk to algae at most sites (Table SI-9, Figure 6). Diuron is a selective herbicide for the control of weeds in sugarcane plantations (Pest Control Products Board 2018). Highest TU values were obtained in sediments from PS59 (TU = 0.24) exceeding ART and PS17 (TU = 0.03) exceeding CRT.
Figure 6: Distribution of risk of toxicity for algae from compounds present in sediment extract based on equilibrium water concentrations. 345-TCP:3,4,5 trichlorophenol; 6:2-FTSA: 6:2-fluorotelomer sulfonic acid