The MAGIC simulation of surface water acidification at, and first year results from, the Bear Brook Watershed Manipulation, Maine, USA

Abstract

The catchments of East and West Bear Brooks, Maine, USA, have been hydrologically and chemically monitored for 3·5 years. Stream chemistries and hydrographs are similar. These clear water streams are low in ANC (0–70 μeq litre −1), with variations caused by changing concentrations of base cations, SO 4, NO 3 and Cl. The latter range between 90–120, 0–40 and 65–75 μeq litre −1, respectively. The West Bear catchment is being treated with six applications per year of dry (NH 4) 2SO 4 at 1800 eq ha −1 year −1. After one year of treatment, the response of the stream chemistry and the response modelled by MAGIC are similar. Retentions of NH 4 and SO 4 are nearly 100% and greater than 80%, respectively. The additional flux of SO 4 is compensated principally by an increased Ca concentration. Episodes of high discharge in the treated catchment are now characterized by lower ANC and pH, and higher Al than prior to the manipulation. Concentrations of NO 3 have increased about 10 μeq litre −1 during the dormant season, presumably due to additional nitrification of N from NH 4. Discharge-chemistry relationships indicate that changes in stream chemistry, except for NO 3, are dominated by ion exchange reactions in the upper part of the soil profile.