ESTIMATING RECHARGE RATES IN A CARBONATE AQUIFER USING RELATIONSHIPS BETWEEN RADIO-ISOTOPIC APPARENT GROUNDWATER AGES AND MG/CA RATIOS

In the Edwards carbonate (Lower Cretaceous) aquifer of the Plateau aquifer system, Texas, groundwater Mg/Ca values are inversely related (r²=0.82) to modern carbon content (¹⁴C; pmc), based on 32 samples. The apparently youngest (1.002 pmc; 3.58 TU ³H) waters have the lowest Mg/Ca ratios (0.17), whereas the oldest (0.0386 pmc; 0 TU ³H) have the highest Mg/Ca ratios (1.14). Groundwater Mg/Ca ratios reflect mixing of younger calcium-carbonate-charged water with older resident groundwater whose elevated Mg/Ca ratios reflect longer residence time in contact with Mg-carbonate. Calculation of ¹⁴C absolute ages is problematic owing to host-rock dissolution. However, sample ¹⁴C- ³H relationships suggest that ¹⁴C apparent age preserves the ranking of samples in order of age; thus, Mg/Ca is an approximate proxy for relative age. This conclusion is supported in the majority of available time-series data by: 1) increases in surface-derived groundwater NO₃ content with Mg/Ca decreases; 2) depleted δ¹³C values (from soil-gas) with young apparent ages; and 3) by increases in SO₄/TDS ratios in older waters with SO₄ probably originating in the sub-Cretaceous Permian evaporite section. Conservative mixing models enable estimation of relative recharge rates by evaluating the fraction of young (<60 yr) groundwater in mixtures whose average age is >60 y. Calculated relative recharge rates are reduced on clay-rich soils that mantle the Plateau crest, and enhanced in recharge windows on karstified clay-poor upper slopes adjacent to the crest. Absolute recharge rates can be calculated on a per-well basis where basic chemical data, mean annual precipitation, and regional recharge rates are available. Limited-area regionalization of per-well values by averaging or by conservative contour mapping reduces variability arising from temporally non-uniform sampling.