Chemical and Isotopic Methods for Quantifying Ground‐Water Recharge in a Regional, Semiarid Environment

Abstract

The High Plains aquifer underlying the semiarid Southern High Plains of Texas and New Mexico, USA was used to illustrate solute and isotopic methods for evaluating recharge fluxes, runoff, and spatial and temporal distribution of recharge. The chloride mass‐balance method can provide, under certain conditions, a time‐integrated technique for evaluation of recharge flux to regional aquifers that is independent of physical parameters. Applying this method to the High Plains aquifer of the Southern High Plains suggests that recharge flux is approximately 2% of precipitation, or approximately 11 ± 2 mm/y, consistent with previous estimates based on a variety of physically based measurements. The method is useful because long‐term average precipitation and chloride concentrations in rain and ground water have less uncertainty and are generally less expensive to acquire than physically based parameters commonly used in analyzing recharge. Spatial and temporal distribution of recharge was evaluated by use of δ2H, δ18O, and tritium concentrations in both ground water and the unsaturated zone. Analyses suggest that nearly half of the recharge to the Southern High Plains occurs as piston flow through playa basin floors that occupy approximately 6% of the area, and that macropore recharge may be important in the remaining recharge. Tritium and chloride concentrations in the unsaturated zone were used in a new equation developed to quantify runoff. Using this equation and data from a representative basin, runoff was found to be 24 ± 3 mm/y; that is in close agreement with values obtained from water‐balance measurements on experimental watersheds in the area. Such geochemical estimates are possible because tritium is used to calculate a recharge flux that is independent of precipitation and runoff, whereas recharge flux based on chloride concentration in the unsaturated zone is dependent upon the amount of runoff. The difference between these two estimates yields the amount of runoff to the basin.

Potential climate change effects on groundwater recharge in the High Plains Aquifer, USA

“…Most of the CHP has a recharge rate of 5-25 mm yr À1 with some higher and lower areas associated with different soil textures. The SHP has an average recharge rate of 10 mm yr À1 , although this occurs predominantly through focused recharge in ephemeral lakes or playas rather than diffuse recharge through the soil matrix (this average recharge rate only applies to part of the SHP as some areas were excluded from the Scanlon et al [10] Recharge focused through playas in the SHP has been reported as 77 mm yr À1 [Wood and Sanford, 1995] and 60-120 mm yr À1 [Scanlon and Goldsmith, 1997], and this occurs over the 1.4% of the area of the SHP covered by playas [Ng et al, 2010]. The remaining 98.6% of the SHP contributed negligible amounts of recharge under native vegetation (<0.1 mm yr À1 [Scanlon and Goldsmith, 1997]).…”

Parameterization and quantification of recharge in crystalline fractured bedrocks in Galicia-Costa (NW Spain)

“…It is also highly soluble and usually has a known marine origin. This technique was widely used, both in the vadose zone and in the saturated-zone (Cook and Böhlke, 2000;Eriksson and Khunakasem, 1969;Sami and Hughes, 1996;Wood and Sanford, 1995). This study uses the CMB approach to evaluate the direct rainfall recharge, using sampling from the saturated-zone.…”

A soil‐water‐balance approach to quantify groundwater recharge from irrigated cropland in the North China Plain

“…Chemical and isotopic tracers have been used successfully not only to help quantify recharge, but also to distinguish between sources (Allison et al, 1994;Gee and Hillel, 1988;Wood and Sanford, 1995). However, tracer data are laborious and expensive to obtain, and thus far are not available for the North China Plain.…”