PUBLIC WATER SUPPLY WELLS AND INDUCED INFILTRATION

When developing a new public water supply well, it is important to consider the origins of the groundwater. A high yielding well in an overburden aquifer adjacent to a surface water feature is likely to withdraw a mixture of groundwater and surface water. When a groundwater well withdraws water from a surface water body it is referred to as induced infiltration. If the well is in close enough proximity to surface water and the aquifer is sufficiently transmissive pathogenic organisms may be able to migrate from the surface water to the wells. This is referred to as groundwater under the direct influence of surface water. In scenarios where this is not the case it is still important to understand where water pumped from wells originates. Large withdrawals of surface water via wells may adversely affect river flows and aquatic life. From a human health perspective, the temperature and chemical composition of surface water is generally more variable than groundwater which may make water treatment more difficult. Further, in developed areas surface water may contain trace levels of pharmaceuticals from wastewater treatment plants. All these factors were considered during a Fall 2023 pumping test of a new public water supply wellfield in central Massachusetts. The pumping test involved withdrawing over 1-million gallons of water per day from an overburden aquifer adjacent to the Blackstone River. Whether or not groundwater withdrawn from the wells was under the direct influence of surface water was evaluated using Microscopic Particle Analysis, which considers the presence of microorganisms and organic debris in water. Induced infiltration was quantified using three methods. (1) The temperature difference between the river and the groundwater was evaluated using measurements taken at the production wells, monitoring wells and river. (2) Oxygen isotope samples were taken throughout the pumping test from the river, production wells and select monitoring wells. Surface water contains lighter isotopes of oxygen than groundwater. (3) Stream gauge/piezometer couplets were installed along the length of the river to measure the reversal of the flow gradient when the Blackstone River began to flow into the underlying aquifer. Finally, the presence of pharmaceuticals in the river and wellfield was considered using caffeine as a proxy.