Fig. 3: Temporal variations in well drilling depths in areas located nearby a well that has been reported to pump fossil water.

The Spearman rank correlation coefficients (ρ) were determined by correlating calendar year versus the fraction of newly constructed wells with depths exceeding the depth of a nearby (<20 km away) groundwater well that is known to pump fossil water. Each panel (a–e) represents correlations completed over one of five different time intervals: 1950–1975, 1975–2000, 2000–2015, 1950–2015 and 1975–2015. Each bar represents the statistical distribution of all study areas' (i.e., 20 km buffers around a groundwater well that has been reported to pump fossil water) correlation coefficients determined within a given aquifer system’s boundaries; the thick horizontal black line represents the median ρ value for all areas with sufficient data within the aquifer system, the top and bottom of the shaded box represents the 25^th-75^th percentile range of ρ values, the dashed line and cap extends to the 10^th-90^th percentile range, and circles represent outlier points. Aquifer systems marked with orange-shaded boxplots have median ρ values exceeding zero (indicative of an increasing proportion of newly drilled wells that are deeper than the well that has been documented to pump fossil water); aquifer systems marked with blue-shaded boxplots have median ρ values of equal to or less than zero (indicative of an unchanging or decreasing proportion of newly drilled wells that are deeper than the well that has been documented to pump fossil water). The number (i.e., text reading: n = x) overlying each box plot represents number of study areas within the aquifer with sufficient data for analyses for a given time interval. We only present box plots for aquifer systems with at least five study areas with sufficient data to determine a rank correlation coefficient. The labels on the x-axis display the title of each aquifer system and the two-letter code for the state that the centroid of the aquifer system lies within (e.g., two-letter code CA denotes that the centroid of the aquifer system lies within California).