Hydrogeology of the Upper Selwyn Catchment

Abstract

Farming practices within the upper Selwyn plains have significantly expanded, and are becoming more dependent on groundwater as a reliable source of irrigation. This expansion has resulted in the rapid development of the groundwater resource and water levels in many wells have reached record low levels. Groundwater resources can be found within at least three aquifers within the glacial gravel deposits of the upper Selwyn plains. Aquifer 1 occurs between approximately 0-30 m, aquifer 2 between 40-85 m and aquifer 3 greater than 100 m below the surface. Aquifers 1 and 2 occur within close proximity to the Selwyn River and its tributaries. Aquifer 1 is unconfined, aquifer 2 semi-confined and aquifer 3 semi-confined to confined. Significant leakage of groundwater occurs between the different aquifers. Recharge sources to the aquifers include rainfall infiltration and river seepage. Water levels and groundwater chemistry suggest that the Selwyn River provides the dominant source of recharge to aquifers 1 and 2 in areas immediately surrounding the river and to the south of the current course of the river between Greendale and Dunsandel.