Chemical and Toxicological Impacts to Cache Slough Following Storm-Driven Contaminant Inputs

Donald Weston,Simone Hasenbein,Kaley Major,Christoph Moschet,Helen Poynton,Nadhirah Johanif,Richard Connon,Thomas Young

doi:10.15447/sfews.2019v17iss3art3

Abstract

Chemical and toxicological testing in the Cache Slough complex (the slough) of the North Delta indicated the aquatic biota are exposed to a variety of wastewater-derived food additives, pharmaceuticals, and personal care products in highest concentration during dry periods, and many insecticides, herbicides and fungicides with peak concentrations after winter rains. The insecticide groups currently known to be of greatest toxicological concern are the pyrethroids and the fiproles (i.e., fipronil and its degradation products). After stormwater runoff enters the system via Ulatis Creek, both pesticide groups attained concentrations that posed a threat to aquatic life. When the commonly used testing species, Hyalella azteca, was placed in Cache Slough, toxicity — and, at times, near total mortality — was seen over at least an 8-km reach of Cache Slough that extended from the uppermost end almost to the junction with the Deep Water Ship Channel. Previous work over many years has shown similar results after other winter storms. However, when H. azteca that carried a mutation providing resistance to pyrethroid pesticides were also deployed in the slough, no ill effects were observed, which provided strong evidence that pyrethroids were responsible for toxicity to the non-resistant strain. Abundant resident H. azteca in Cache Slough carry any of four mutations that provide resistance to pyrethroids. They also carry a mutation that provides resistance to organophosphate pesticides, and likely carbamate pesticides as well. After many years of exposure, sensitive genotypes have been nearly eliminated from the system, and replaced by a population unaffected by many insecticides now in common use. We offer a variety of reasons why this shift to a population with mutant genotypes is of considerable concern, but also note that society has yet to fully consider the ecological and regulatory ramifications of the evolutionary attainment of pollutant resistance.

Highlights

The Cache Slough complex in the northwest Sacramento-San Joaquin Delta is a network of tidal freshwater sloughs bounded by levees, with the surrounding uplands used for irrigated agriculture
Two days after the rain ceased, Ulatis Creek continued to carry elevated total suspended solids (TSS) concentrations, though turbidity was substantially reduced from the earlier peak, and the input was not sufficient to affect anything but the uppermost reaches of the Slough
The aquatic biota of the Cache Slough complex live within two alternating chemical environments

Summary

Introduction

The Cache Slough complex (the slough) in the northwest Sacramento-San Joaquin Delta is a network of tidal freshwater sloughs bounded by levees, with the surrounding uplands used for irrigated agriculture. The area is of particular significance as habitat for the Delta Smelt (Hypomesus transpacificus), an endemic fish that has suffered dramatic declines in numbers, and is listed as endangered under the California Endangered Species Act and as threatened under comparable federal legislation (CDFW 2019). In view of the significance of Cache Slough and associated waterways, it is concerning that its waters have been shown to be toxic to a commonly used testing organism, the amphipod, Hyalella azteca. The same study documented one occasion when the organophosphate pesticide, chlorpyrifos, entered nearby Lindsey Slough via agricultural return flow from surrounding croplands, and caused mortality in H. azteca when slough waters were tested

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