The influence of catchment characteristics, agricultural activities and atmospheric deposition on the chemistry of small streams in the English Lake District

Abstract

Six synoptic surveys of 55 streams draining the central English Lake District, Cumbria, were carried out during the period May 1996–March 1997 to measure streamwater chemistry under a variety of flow conditions and relate this to geology and land use. Annual mean flow-weighted Gran alkalinity of the streams in the study ranges from 2 to 452 μeq l−1. Using 200 μeq l−1 alkalinity as a sensitivity threshold, 41 of the streams (75%) can be considered sensitive to acidification via acid deposition. Of the sensitive streams, 12 have annual mean flow-weighted Gran alkalinities less than 50 μeq l−1, suggesting a critical level of sensitivity to acidification. By grouping the streams into categories based on the dominant geology, it is clear that geology plays a major role in determining streamwater chemistry. However, the chemical signature of the streams is significantly modified through the effects of land use. One example is stream nitrate (NO3−) concentration which ranges from 0 to 238 μeq l−1 (flow-weighted mean). All of the seven streams with flow-weighted NO3− values above 50 μeq l−1 drain predominantly agricultural catchments, with cover varying between 63 and 100%. The flow-weighted chemistry data were related to catchment characteristics (geology, land use, deposition and soil type) using multiple regression analysis. The influence of agriculture generally dominated the models. In particular, strong relationships were found between percentage agricultural land and Gran alkalinity (R2=0.53), percentage agricultural land and calcium (R2=0.50) and percentage agricultural land and nitrate concentrations (R2=0.40). This survey confirms that the resistant bedrock of the Lake District generally results in low base cation resupply to the streams. Due to this very low buffering capacity it is likely that many of the streams have been impacted by acid deposition, especially those draining the slow-weathering Skiddaw Slates, Borrowdale Volcanics and granitic bedrock. The easier-weathering Silurian Slates provide higher buffering capacity to drainage waters. Agriculture, which is mostly found on the Silurian Slates, increases the buffering capacity of the drainage water but also results in enhanced nitrate leaching.