The concentration of organochlorine pesticides were below the detection limit (0.005 mg/L) at all the sampling sites. Meanwhile, Ntow (
2005) reported gamma-HCH levels of 8 µg/L as well as alpha-endosulfan and endosulfan sulfate concentrations of 36 and 23 µg/L respectively in the Volta Lake. Settling of agricultural chemicals along with sediment could explain the low water concentrations. A study by Logah et al. (
2017) suggests that there is high sediment concentrations downstream of Akuse which can be attributed to sand mining activities at various sections of the river and sediment input from tributaries could explain the low concentrations of agricultural pesticides in the water. Also, the absence of detection of organochlorines could be due to the ban of the use of e.g. DDT (GEPA Ghana Environmental Protection Agency (
2008)) in Ghana, over time leading to possible degradation and dilution in the water body. Recent use of such products may also have been stopped which would have lowered the organochlorine pesticide levels. However, λ-cyhalothrin was detected at Sedorm 1 and Akuse Canal in the dry season in concentrations of 0.6 and 8.8 µg/L respectively. Cypermethrin was detected at a concentration of 1.4 µg/L at Marine during the January–February dry season sampling period. λ-cyhalothrin is highly lipophilic and tends to bind rapidly and strongly to organic materials (Maund et al.
1998; Leistra et al.
2003). Furthermore λ-cyhalothrin is highly toxic to some groups of aquatic organisms, particularly insects and crustaceans, with the midge
Chaoborus obscuripes being sensitive (48- and 96-h EC
50 = 0.0028 µg/L. Other insect larvae (Hemiptera, Ephemeroptera) and macrocrustacea (Amphipoda, Isopoda) are also relatively sensitive, with 48-and 96-h EC
50 values between 0.01 and 0.1 µg/L (Schroer et al.
2004). Likewise, fish is sensitive with reported LC
50 in bluegill sunfish, 0.21 μg/L; rainbow trout, 0.24 µg/L (Kidd and James
1991). Cypermethrin likewise is very highly toxic to fish and aquatic invertebrates. The LC
50 (96-h) for cypermethrin and rainbow trout is 8.2 µg /L, and for bluegill sunfish is 1.8 µg/L while the effect concentrations for the total crustacean community and cladoceran and copepod subgroups in a study by Friberg-Jensen et al. (
2003) ranged between 0.02–0.07 and 0.04–0.17 µg/L, respectively, with copepods being less sensitive than cladocerans. This raises concern as based on intrinsic sensitivity, biological traits, mode of action used for invertebrate vulnerability index rankings by Rico and Van den Brink (
2015), Ephemeroptera, Plecoptera, Tricoptera, and Odonata genera were identified potentially most vulnerable to pyrethroids in aquatic ecosystems. The pesticide data were however not analysed further due to the low number of detections (Table SI 4).