How much a reservoir affects groundwater will depend on its size, location, and the nature of the underlying geology. When a reservoir is filled, it can lead to increased water pressure in the underlying aquifers, causing groundwater levels to rise. This can be beneficial for local wells and aquifer recharge. However, the impoundment of water in the reservoir can also lead to the accumulation of sediments and organic matter, potentially impacting groundwater quality.
Furthermore, alterations in surface water flow can influence the movement of groundwater, potentially leading to changes in the direction of groundwater flow and affecting the availability of groundwater resources. The impact on groundwater varies depending on local hydrogeological conditions and reservoir characteristics.
