Widespread and increased drilling of wells into fossil aquifers in the USA

Abstract

Most stored groundwater is ‘fossil’ in its age, having been under the ground for more than ~12 thousand years. Mapping where wells tap fossil aquifers is relevant for water quality and quantity management. Nevertheless, the prevalence of wells that tap fossil aquifers is not known. Here we show that wells that are sufficiently deep to tap fossil aquifers are widespread, though they remain outnumbered by shallower wells in most areas. Moreover, the proportion of newly drilled wells that are deep enough to tap fossil aquifers has increased over recent decades. However, this widespread and increased drilling of wells into fossil aquifers is not necessarily associated with groundwater depletion, emphasizing that the presence of fossil groundwater does not necessarily indicate a non-renewable water supply. Our results highlight the importance of safeguarding fossil groundwater quality and quantity to meet present and future water demands.

Modern groundwater reaches deeper depths in heavily pumped aquifer systems

“…We identified n = 17 aquifer systems where our approach (i.e., calculating the depth below which 60, 70 or 80% of samples contain <25% modern groundwater) provided an imperfect estimate of the depth below which modern groundwater becomes scarce due (a) a lack of groundwater tritium data at depth, or (b) a lack of a consistent decline in modern groundwater fractions with depth (Supplementary Table 1 and Supplementary Figs. 4,6,9,12,16,19,33,37,44,52,54,59,71,79,82,87,88). We excluded these n = 17 aquifer systems from our analyses.…”

“…Light grey polygons underlying the yellow-red points are aquifer system boundaries published by GebreEgziabher et al (ref. 19 ; data available via https://www.hydroshare.org/resource/d2260651b51044d0b5cb2d293d21af08/ ). b – o display hydrostratigraphy via cross-sections for n = 14 of the n = 74 aquifer systems that we studied.…”

“…Aquifer system boundaries from ref. 19 are available to download via CUAHSI HydroShare: https://www.hydroshare.org/resource/d2260651b51044d0b5cb2d293d21af08/ . Groundwater withdrawal estimates (county-scale) are available via https://pubs.er.usgs.gov/publication/cir1441 .…”

“…After completing our 3 H-based calculations of the proportion of individual well water samples comprised of modern groundwater, we grouped wells located within the same aquifer system (US aquifer geospatial data from ref. 19). These two-dimensional aquifer system areas are underlain by multiple geologic formations of varying hydrogeologic characteristics (for hydrogeologic cross sections for each study area, see Supplementary Figs.…”

Investigation and Estimation of Groundwater Level Fluctuation Potential: A Case Study in the Pei-Kang River Basin and Chou-Shui River Basin of the Taiwan Mountainous Region

“…Due to the frequent drought in many countries worldwide, groundwater resources stored in fractured rock masses in mountainous areas have gradually become alternative water resources to combat 2 of 19 this natural disaster [3,4]. In addition, much of the groundwater, either recent anthropocene groundwater [5] or older fossil groundwater [6], stored under porous and fractured rocks is vulnerable to water quality deterioration and groundwater depletion at geographically varying locations. Therefore, assessing the groundwater resource potential and quality in mountainous areas is a prerequisite.…”