Evaluation of three experimental low-technology approaches to acid mitigation in headwater streams

Abstract

Although powered dosing devices may be effective in regulating pH and acid neutralizing capacity in acid-sensitive streams, their high costs and maintenance requirements may make them unsuitable for most sites. Three alternative approaches were tried on a mountain trout stream with pH 4.5 to 5.3 and essentially no acid neutralizing capacity: (1) A barrier of wire gabions across the stream, filled with limestone chunks, raised pH about 0.5 unit for about 2 weeks. Its effect then became undetectable. Effectiveness could have been improved by using high-Ca (>95%) stone of more uniform size (5 cm in diameter), and improving the control of leaf accumulation. No biological effects of the device were detected. (2) Metal mesh baskets fed by gravity with soda ash (sodium carbonate) briquettes worked well in raising pH by as much as 1.5 units and acid neutralizing capacity by as much as 40 ueq.L⁻¹ for several months. However, it proved difficult to provide adequate replenishment to keep these flow-regulated devices operating during high-flow periods. (3) To reduce consumption of soda ash, we tested devices that dissolved the briquettes in an enclosed chamber and fed the concentrated sodium carbonate solution into the stream. These provided adequate neutralization when operating properly but frequently failed due to pipe clogging and to the hardening of the briquettes into a solid mass. Studies of benthic invertebrate drift and abundance associated with the soda ash treatment showed no significant effect in a few hours or in 60 days.