THE INFLUENCE OF CATCHMENT CHARACTERISTICS, AGRICULTURAL ACTIVITIES AND ATMOSPHERIC DEPOSITION ON THE CHEMISTRY OF SMALL STREAMS IN THE ENGLISH LAKE DISTRICT

Six synoptic surveys of 55 streams draining the central English Lake D istrict, Cumbria, were carried out during the period May 1996-March 19 97 to measure streamwater chemistry under a variety of flow conditions and relate this to geology and land use. Annual mean flow-weighted Gr an alkalinity of the streams in the study ranges from 2 to 352 mu eq l (-1). Using 200 mu eq 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-we ighted Gran alkalinities less than 50 mu eq l(-1), suggesting a critic al level of sensitivity to acidification. By grouping the streams into categories based on the dominant geology, it is clear that geology pl ays a major role in determining streamwater chemistry. However, the ch emical signature of the streams is significantly modified through the effects of land use. One example is stream nitrate (NO3-) concentratio n which ranges from 0 to 238 mu eq l(-1) (flow-weighted mean). All of the seven streams with flow-weighted NO3- values above 50 mu eq l(-1) drain predominantly agricultural catchments, with cover varying betwee n 63 and 100%. The flow-weighted chemistry data were related to catchm ent characteristics (geology, land use, deposition and soil type) usin g multiple regression analysis. The influence of agriculture generally dominated the models. In particular, strong relationships were found between percentage agricultural land and Gran alkalinity (R-2 = 0.53), percentage agricultural land and calcium (R-2 = 0.50) and percentage agricultural land and nitrate concentrations (R-2 = 0.40). This survey confirms that the resistant bedrock of the Lake District generally re sults 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 im pacted by acid deposition, especially those draining the slow-weatheri ng Skiddaw Slates, Borrowdale Volcanics and granitic bedrock. The easi er-weathering Silurian Slates provide higher buffering capacity to dra inage waters. Agriculture, which is mostly found on the Silurian Slate s, increases the buffering capacity of the drainage water but also res ults in enhanced nitrate leaching. (C) 1998 Elsevier Science B.V.