Human activities such as coal mining, oil-drilling and surface mining can frequently lead to contamination of surface water in surrounding areas. Mountaintop removal-valley fill coal mining, a type of surface mining, has been shown to adversely affect the chemical composition of freshwater streams and, more importantly, populations of aquatic insects that dwell in these streams. Residential aquatic insects are used as biomonitiors of water quality because they are the dominant invertebrate faunal groups in streams. These streams that are impacted by valley-filling display elevated levels of selenium, total dissolved solids (TDS) and pH. Several studies have examined the effects of selenium in a representative aquatic insect, reporting reduced survivorship, body mass, and fecundity after dietary exposure. Recently, studies have shown that increased concentrations of TDS with mountaintop mining ionic signatures were toxic to the mayfly Centroptilum triangulifer suggesting that high TDS water can contribute to population declines in aquatic insects. In our lab, we are focused on understanding the toxicity of individual ion components of high TDS water (specifically [SO42-] and [HCO3-]) to see if any one ion is the primary driver of toxic effects. We have conducted 48- and 96-hour acute toxicity test with newly hatched C. triangulifer larvae, exposed to a range of [SO42-] and [HCO3-] while holding all other ion concentrations constant, in order to obtain LC50 values for the organism. We also investigated the hemolymph osmolarity of an aquatic insect after exposure to high TDS water. Toxicity testing on SO42- and HCO3 is ongoing .