Pumping Alkaline Groundwater to Restore a Put-and-Take Trout Fishery in a Stream Acidified by Atmospheric Deposition

Abstract

The purpose of this study was to pump alkaline groundwater into Linn Run in southwestern Pennsylvania to neutralize acidity, reduce dissolved aluminum concentrations, and protect stocked brown trout Salmo trutta and brook trout Salvelinus fontinalis during the springs of 1985 and 1986. A regression model was used to estimate the groundwater volume needed to maintain predetermined stream pH levels for a given streamflow. Equal numbers of brook and brown trout were stocked in a treatment stream section influenced by the wells and a naturally alkaline downstream reference section before and during the fishing season each year. Groundwater addition increased mean stream pH from 4.9 upstream of the wells to 6.0 in the treatment section and reduced mean dissolved aluminum from 0.36 mg/L to 0.03 mg/L above and below the wells, respectively. A large runoffepisode following the 1985 preseason stocking overwhelmed the capacity of the wells to neutralize Linn Run, causing at least 25% mortality and reducing the catch rate of brown trout in the treatment section. No mortality of caged trout occurred in the treatment section except during this large runoff event. However, mortality was rapid for caged trout upstream of the wells; median survival times were 67 and 29 h for brook and brown trout, respectively. Otherwise, angler catch rates and trout harvest were not significantly different between the treated and reference sections. Electrofishing surveys revealed greater downstream than upstream movement of stocked trout, which may indicate a response to water chemistry. Alkaline groundwater additions also caused an increase in the density of native brook trout in the treatment section via upstream migration from downstream refuges influenced by groundwater.