Modeling the influence of land use on groundwater chloride loading to lakes

Concern over potential ecological impacts of rising chloride concentrations in lakes has intensified interest in predicting how chloride concentrations in lakes will change over time. Since the 1970s, chloride concentrations have risen in a group of central Wisconsin lakes. The principal source of water to these lakes is groundwater, and rising chloride concentrations have coincided with increased chloride use within the groundwater contributing areas. A groundwater lake loading model was developed to show how chloride sources in groundwater-contributing areas change chloride concentrations in lakes. By using this model with measured chloride concentrations from the lakes, the chloride exported from different land uses was estimated. Chloride export from agricultural and residential land uses was estimated to be 41 kg/ha.yr and 51 kg/home.yr, respectively. Highway deicing was determined to be a minor source of chloride to these lakes due to the low density of primary roads. Chloride concentrations will increase in these lakes until they are at steady-state with respect to chloride loading. The groundwater lake loading model showed that concentrations in these lakes average 50% of their predicted steady-state concentrations, but individually they range from <10% to >90% of their ultimate concentrations.