Watershed Acidification Model And The Soil Acid Neutralization Capacity Concept

Abstract

<p>A computer simulation model is developed and tested which is capable of simulating the movement of water and chemical constituents, specifically H⁺, through a watershed.</p> <p>Emphasis is placed on the interaction of H⁺ with the soil system. An acid titration method has been developed to determine the acid neutralization capacity (ANC) of bulk soil or soil minerals. ANC functions have been developed which consider the effect of H⁺ - cation exchange, H⁺ - Al⁺³ exchange and solution reactions at low pH levels (< pH 4.5). The ANC and ANC function concepts developed in this research are more relevant to acidic loadings on soils than cation exchange capacity or base saturation.</p> <p>The model operates on a daily time step and the input data required to drive the model are: 1) precipitation depth 2) mean air temperature 3) mean relative humidity and 4) precipitation pH and alkalinity/acidity.</p> <p>The model has been applied to a calibrated watershed, Batchawana Lake Basin, the headwater basin of the Turkey Lakes Watershed, located approximately 50 km north of Sault Ste. Marie, Ontario. The model has been found to be capable of accurately predicting all of the hydrologic flows for the Batchawana Lake Basin and should be applicable for most small (< 5 km²) non-urban watersheds. The model also accurately predicts the daily lakewater pH values, snowpack and snowmelt chemistry and average stream-water pH values. The ANC functions used in the model are not designed to predict short-term fluctuations in soilwater pH values which are normally due to biological processes but rather long-term changes in soilwater pH levels due to long-term inputs of acidic precipitation.</p> <p>Sensitivity analyses were carried out on the model in order to determine the key parameters controlling the rate of acidification of a watershed. In addition to the actual acidic loading rate, the more sensitive parameters in decreasing significance are:</p> <p>i) -infiltration rates ]</p> <p>ii) -soil depth ]</p> <p>both control the "degree of interaction" of the incident acidity with the soil materials</p> <p>iii) -soil horizon ANC's</p>