Effects of Seasonal Operation on the Quality of Water Produced by Public‐Supply Wells

Abstract

Seasonal variability in groundwater pumping is common in many places, but resulting effects of seasonal pumping stress on the quality of water produced by public-supply wells are not thoroughly understood. Analysis of historical water-quality samples from public-supply wells completed in deep basin-fill aquifers in Modesto, California (134 wells) and Albuquerque, New Mexico (95 wells) indicates that several wells have seasonal variability in concentrations of contaminants of concern. In Modesto, supply wells are more likely to produce younger groundwater with higher nitrate and uranium concentrations during the summer (high) pumping season than during the winter (low) pumping season. In Albuquerque, supply wells are more likely to produce older groundwater with higher arsenic concentrations during the winter pumping season than during the summer pumping season. Seasonal variability in contaminant concentrations in Modesto is influenced primarily by effects of summer pumping on vertical hydraulic gradients that drive migration of shallow groundwater through the aquifer to supply wells. Variability in Albuquerque is influenced primarily by the period of time that a supply well is idle, allowing its wellbore to act as a conduit for vertical groundwater flow and contaminant migration. However, both processes are observed in each study area. Similar findings would appear to be likely in other alluvial basins with stratified water quality and substantial vertical head gradients. Results suggest that even in aquifers dominated by old groundwater, changes to seasonal pumping patterns and/or to depth of well completion can help reduce vulnerability to selected contaminants of either natural or anthropogenic origin.

A Ternary Age‐Mixing Model to Explain Contaminant Occurrence in a Deep Supply Well

“…As stated above, water‐level gradients and flowmeter results show that upward gradients exist in the well under ambient conditions, allowing deep groundwater to migrate upward through the wellbore (or annular material) and move out into the aquifer at intermediate depths (approximately 110 to 150 mbls). This process results in the storage of deep groundwater at intermediate depths until pumping resumes (see Bexfield and Jurgens, ). Consequently, when pumping resumes, the well receives groundwater from the deep part of the aquifer and deep groundwater stored at intermediate depths.…”

“…The concentrations of 3 H, CFC-113, 14 C, and arsenic in water from the well vary according to the time of year (or pumping season) when the well is sampled (Table 1) During prolonged periods of nonpumping, waterlevel gradients and flowmeter results show that upward gradients in the well allow deep groundwater to migrate upward through the wellbore (or annular material) and move out into the aquifer at intermediate depths (approximately 110 to 150 mbls; Figure 3). This process results in the storage of deep groundwater at intermediate depths until pumping resumes (see Bexfield and Jurgens, 2014). Consequently, when the well is sampled after long periods of nonpumping, concentrations of 3 H, CFC-113, and 14 C and the number of VOCs were lower and arsenic concentrations were higher because the well received groundwater from the deep part of the aquifer plus deep groundwater stored at intermediate depths while the well was idle.…”

Inactive supply wells as conduits for flow and contaminant migration: conditions of occurrence and suggestions for management

“…1) acted as a conduit partly based on (1) local downward vertical hydraulic gradients and (2) correlation between periods of pump inactivity and concentration increases in deeper water-bearing zones of constituents typically found in shallow water-bearing zones. Bexfield et al (2012), Bexfield and Jurgens (2014), Jurgens et al (2008) and (2014) documented vertical groundwater flow and contaminant migration through municipal supply wells in two different parts of USA (Modesto, California and Albuquerque, New Mexico) during periods of nonoperation. Ambient flow was downwards in one well and upwards in the other.…”

Identifying areas of degrading and improving groundwater-quality conditions in the State of California, USA, 1974–2014

“…Land use practices can also alter the natural chemistry of water that recharges an aquifer and cause trace elements that are naturally present, like uranium, to become mobilized (Jurgens et al 2010;Ayotte et al 2011). Shortterm, cyclical pumping patterns resulting from semiannual water demand can also lead to seasonal waterquality variations in wells (Bexfield and Jurgens 2014). On longer time scales, groundwater-quality trends may be caused by regional pumping patterns that alter the origin of groundwater reaching wells (Starn et al 2014).…”