THE EXPERIMENTAL WATERSHED LIMING STUDY - COMPARISON OF LAKE AND WATERSHED NEUTRALIZATION STRATEGIES

The Experimental Watershed Liming Study (EWLS) was initiated to evalua te the application of CaCO3 to a forested watershed in an effort to mi tigate the acidification of surface water. The objective of the EWLS w as to assess the response of the Woods Lake watershed to an experiment al addition of CaCO3. During October 1989, 6.89 Mg CaCO3/ha was applie d by helicopter to two subcatchments comprising about 50% (102.5 ha) o f the watershed area. The EWLS involved individual investigations of t he response of soil and soil water chemistry, forest and wetland veget ation, soil microbial processes, wetland, stream and lake chemistry, a nd phytoplankton and fish to the CaCO3 treatment. In addition, the Int egrated Lake/Watershed Acidification (ILWAS) model was applied to the site to evaluate model performance and duration of the treatment. The results of these studies are detailed in this volume. The purposes of this introduction and synthesis paper are to: 1) present the overall d esign of the EWLS, 2) discuss the linkages between the individual stud ies that comprise the EWLS, and 3) summarize the response of the lakew ater chemistry to watershed addition of CaCO3 and compare these result s to previous studies of direct lake addition. An analysis of lake che mistry revealed the watershed treatment resulted in a gradual change i n pH, acid neutralizing capacity (ANC) and Ca2+ in the water column. T his pattern was in contrast to direct lake additions of CaCO3, which w ere characterized by abrupt changes following base addition and subseq uent rapid reacidification. Over the three-year study period, the supp ly of ANC to drainage waters was largely derived from dissolution of C aCO3 in wetlands. Relatively little dissolution of CaCO3 occurred in f reely draining upland soils. The watershed treatment had only minor ef fects on forest vegetation. The watershed treatment eliminated the epi sodic acidification of streamwater and the near-shore region of the la ke during snowmelt, a phenomenon that occurred during direct lake trea tments. Positive ANC water in the near-shore area may improve chemical conditions for fish reproduction, and allow for the development of a viable fish population. The watershed CaCO3 treatment also decreased t he transport of Al from the watershed to the lake, and increased the c oncentrations of dissolved organic carbon (DOG) and dissolved silica ( H4SiO4) in stream and lakewater. The watershed treatment appeared to e nhance soil nitrification, increasing concentrations of NO3- in soilwa ter and surface waters. However, the acidity associated with this NO3- release was small compared to the increase in ANC due to CaCO3 additi on and did not alter the acid-base status of Woods Lake. Acid neutrali zing capacity (ANC) budgets for 12-month periods before and after the watershed treatment showed that the lake shifted from a large source o f ANC to a minor source due to retention of SO42-, NO3-, Al and the el evated inputs of Ca2+ associated with the watershed CaCO3 application. In contrast to the direct lake treatments, Ca2+ inputs from the water shed application were largely transported from the lake.