Evaluation of the performance of multiple-well hydraulic barriers on enhancing groundwater extraction in a coastal aquifer

Abstract

Hydraulic barriers using either two or three injection wells are studied analytically to evaluate their performance on improving groundwater extraction in a coastal aquifer under the threat of seawater intrusion. The multiple well system is operated by extracting fresh groundwater through a landward well and partially reinjecting into the aquifer through two or three wells located parallel to the coastline and between the interface toe and the extraction well. We found that with appropriate well locations and pumping rates, the multiple-well system could outperform a single extraction well in terms of net extraction rate, which is up to 90% higher for an aquifer with a thickness of 20 m, hydraulic conductivity of 10 m/d, and freshwater influx of 0.24 m2/d. Sensitivity analyses reveal that the monotonicity of the maximum net extraction rate is determined by the total injection rate. In specific, when the total injection rate is fixed, the maximum net extraction rate is a non-monotonic function of the distance between the injection well and the coastline and the distance between two injection wells, resulting in a global maximum. When the total injection rate is not fixed, by contrast, the maximum net extraction rate depends on the total injection rate, and a larger total injection rate leads to a larger maximum net extraction rate. By using different injection rates for side and middle injection wells of the three-injection-well hydraulic barrier, the maximum net extraction rate could be further improved, depending on the distance between the two injection wells. Analytical analyses can provide initial guidance for the design of a multiple-well system in coastal aquifers with the purpose of obtaining enhanced extraction under the threats of seawater intrusion.