Episodic response of aluminum chemistry in an acid‐sensitive Massachusetts catchment

A field investigation was conducted to evaluate factors that regulate episodic variations of Al chemistry in an acid‐sensitive Massachusetts catchment. Stream water was collected from an upland subcatchment, a wetland subcatchment, and below the confluence of these two drainage areas in the West Wachusett Brook at 6 hour intervals during the first 2 weeks of November 1985. All major solutes and three Al fractions were analyzed. During the monitoring period, two different rainfall events of contrasting origin (i.e., a coastal storm and an inland storm) occurred. Rainfall volume, intensity, and duration were important factors regulating short‐term acidification of this stream. Ion exchange processes rather than mineral phase solubility appear to control stream water Al concentrations during high‐flow periods. The episodic acidification and subsequent release of Al in the upland stream were associated with a decrease in basic cations. In contrast, Al chemistry from the wetland drainage was regulated by the release of organic acids which served as mobile anions and were responsible for chelating and transporting Al. Chemical equilibrium calculations showed that waters draining the upland subcatchment were potentially toxic to fish during high‐flow periods, whereas the wetland stream exhibited minimal potential for Al toxicity.