The effects of geomorphology and seasonality on the chemistry of carbonate groundwater

The equilibrium chemistry of carbonate groundwater at the regional and long-term scales of aggregation is affected by geomorphic factors and by the seasonality of temperature and recharge regimes. The general trend of Ca concentration differences between regions is governed by annual mean air or groundwater temperature and by the nature of the solution system. Higher concentrations result from the coincident system where carbonate material, soil air and water are all present together and lower concentrations from the sequential system where carbonate material occurs downstream of the soil air. Coincident systems occur in areas where carbonate material is present in the regolith (either by natural or human activity), where a porous or friable bedrock lies at shallow depth or where soils are shallow. Sequential systems occur where there is a deep non-carbonate regolith, where there is a non-carbonate cap to the carbonate bedrock or where there is a high water table. Reductions in equilibrium Ca concentration can be caused by low soil-air volumes relative to percolation volume. This may be particularly significant in cool regions, where soils are shallow and clayey, and where recharge occurs in short high-intensity events.