Temporal Changes in the Vertical Distribution of Flow and Chloride in Deep Wells

Abstract

The combination of flowmeter and depth-dependent water-quality data was used to evaluate the quantity and source of high-chloride water yielded from different depths to eight production wells in the Pleasant Valley area of southern California. The wells were screened from 117 to 437 m below land surface, and in most cases, flow from the aquifer into the wells was not uniformly distributed throughout the well screen. Wells having as little as 6 m of screen in the overlying upper aquifer system yielded as much as 50% of their water from the upper system during drought periods, while the deeper parts of the well screens yielded 15% or less of the total yield of the wells. Mixing of water within wells during pumping degraded higher-quality water with poorer-quality water from deeper depths, and in some cases with poorer-quality water from the overlying upper aquifer system. Changes in the mixture of water within a well, resulting from changes in the distribution of flow into the well, changed the quality of water from the surface discharge of wells over time. The combination of flowmeter and depth-dependent water quality data yielded information about sources of high-chloride water to wells that was not available on the basis of samples collected from nearby observation wells. Changing well design to eliminate small quantities of poor-quality water from deeper parts of the well may improve the quality of water from some wells without greatly reducing well yield.

Modeling Effects of Multinode Wells on Solute Transport

“…If the withdrawal (or injection) rate specified for a pumping well is insufficient to induce unidirectional flow between all well nodes and the corresponding aquifer cells, the resulting complex intraborehole flow pattern can generate a nonuniform distribution of solute concentrations in the borehole, similar to that in a nonpumping multinode well. For example, Izbicki et al (2005) collected depth‐dependent water quality samples in a well under pumping conditions and demonstrated large changes in chloride concentration with depth. Therefore, the assumption of complete mixing in a pumped borehole may not be valid, and water entering the ground water system from a node in a withdrawal well may not have the same solute concentration as water discharging through a pump at the wellhead.…”

River-Induced Flow Dynamics in Long-Screen Wells and Impact on Aqueous Samples

“…Elci et al (2001) summarized borehole flowmeter test results on a total of 142 wells at 16 sites in 12 states that demonstrated a measurable amount of wellbore flow in 73% of the cases. Others have identified ambient vertical flows in long‐screen wells based on observed vertical distributions in contaminant concentration and field parameters (Hutchins and Acree 2000; Izbicki et al 2005; Corcho Alvarado et al 2009; McDonald and Smith 2009). In addition to these field investigations, several modeling studies (Reilly et al 1989; Reilly and LeBlanc 1998; Elci et al 2001, 2003) have been conducted to assess the impacts of wellbore flow on measured concentrations in long‐screen wells and the redistribution of contaminants within the aquifer.…”

Salinity of deep groundwater in California: Water quantity, quality, and protection

“…Typically, groundwater salinity increases with depth (16). Fresh groundwater resources occurring at relatively shallow depths ( 1,000 m) have been studied extensively in terms of groundwater availability (17)(18)(19)(20)(21)(22) and quality (23)(24)(25)(26). In California, water quality data from over 200,000 groundwater wells are available from the State Water Resources Control Board Groundwater Ambient Monitoring and Assessment Program (27).…”