Solder in the USGS ISR 2020-5050 reports that groundwater susceptibility to contamination was investigated by using environmental tracer-based groundwater age metrics in the south Atlantic and Gulf Coast principal aquifer systems of the Southeastern Coastal Plain, Mississippi embayment–Texas coastal uplands, and the Coastal Lowlands. Samples of dissolved gas, tritium, sulfur hexafluoride, tritiogenic helium, and carbon-14 were collected from 231 public supply wells in the 3 principal aquifer systems.
Dissolved gas models were used to characterize recharge conditions and they identified recharge mechanisms that ranged from rapid, but short-lived, water table rises (possibly associated with large scale flooding), to slower diffuse recharge not associated with large water table fluctuations.
Dissolved gas and geochemical correction models were used to calculate and (or) correct tracer concentrations before input to lumped parameter models of groundwater age. Lumped parameter models that were fit to tracer concentrations indicated groundwater was relatively old across the aquifer systems, with an estimated mean age of about 30,000 years. Estimates of groundwater age were related to hydrogeology, with increasing groundwater ages associated with greater depth, confinement, and distance from the recharge zone.
Young groundwater with mean ages less than 2,000 years generally was in unconfined parts of the aquifer system, except for local areas of heavy groundwater extraction from unconfined aquifer units where estimated mean ages were up to 15,000 years. Lumped parameter model optimized age distributions describe the relative contribution of differing flow paths to the mean age, and a composite distribution of all samples from the three aquifer systems indicated that about 15 percent of the total sampled water had an age of less than 100 years.
Various metrics of susceptibility, to land surface and geogenic contamination sources, derived from the age distributions, indicated geogenic sources as the primary threat to groundwater quality in the aquifer systems. Values of the susceptibility index (unitless) and fraction of recharge since 2,000 and 15,000 years before present are provided for assessment of individual well susceptibility. The data and interpretation methods presented here provide an additional means of investigating the susceptibility and sustainability of groundwater resources of the Southeastern Coastal Plain, Mississippi embayment–Texas coastal uplands, and the Coastal Lowlands aquifer systems.
The author presented the tracer-based estimates of mean age, age distributions, and susceptibility metrics for 231 wells in:
- the south Atlantic and Gulf Coast principal aquifer systems (Southeastern Coastal Plain, SECP;
- Mississippi embayment–Texas coastal uplands, METX; and
- Coastal Lowlands, CLOW) are presented in this report.
As might be expected for a large study area that covered approximately 4.17x105 square miles in parts of 12 states, a wide range of groundwater age and susceptibility was observed. Estimates of mean groundwater ages ranged from 17 years to greater than 50,000 years. In general, groundwater across the three aquifer systems is dominated by pre-modern water (207 of the 231 total samples) with an overall mean age of about 33,000 years. Estimated groundwater ages are youngest at the up-dip extent of the aquifer systems where the aquifer units crop out and increase in age with increasing depth, confinement, and distance from the outcrop. In the SECP, with relatively continuous confining units, groundwater age has a coherent spatial structure with younger groundwater along the in-land extent of the aquifer system and the oldest groundwater located in the furthest down-dip portions of the aquifer system.
In the CLOW and METX, with a more complex hydrogeology of discontinuous confining units and steeply dipping aquifer units, groundwater ages are more spatially variable with young groundwater ages (less than 2,000 years) interspersed with groundwater greater than 10,000 years old. Estimated groundwater age also is related in the regional climate and groundwater extraction rates. The relation between age and climate is best illustrated by the less frequent occurrence of groundwater less than 2,000 years old in the more arid western portions of the METX and CLOW; this is indicative of lower recharge rates compared to aquifer-recharge areas in the three aquifer systems that receive more precipitation. The impact of heavy pumping on groundwater age might be apparent in the eastern portions of Texas near the Houston metropolitan area. This region of east Texas generally has a similar climate to that of South Carolina and eastern Georgia but with markedly older groundwater in the unconfined up-dip portions of the respective aquifers. The increased groundwater ages can be related to groundwater extraction rates in excess of recharge rates resulting in capture of older groundwater from storage to meet pumping demands.
The intrinsic susceptibility across the aquifer system was estimated based on metrics which summarize the groundwater age distributions. Separate metrics, such as the susceptibility index (SI) for assessing susceptibility to land-surface contamination and the fraction of recharge less than 15,000 years old for assessing susceptibility to geogenic contamination, were calculated for individual and composite age distributions for each of the three aquifer systems. Individual SI generally was low with a median value of about 0.03 indicating that groundwater susceptibility to land surface contamination generally was not a concern across the three aquifer systems except at specific sites.
Conversely, the fraction of recharge less than 15,000 years old also was relatively low with an average value of 0.43, meaning 57 percent of the total groundwater sampled was greater in age than 15,000 years and implies that contamination of groundwater from geogenic sources is likely a greater concern than anthropogenic contamination. A third group of samples, containing a mixture of young water susceptible to land-surface contamination and old groundwater susceptible to geogenic contamination sources, were difficult to identify in this study because of modern tracer contamination (sulfur hexafluoride) and calculation uncertainty (tritiogenic helium-3). Wells that were sampled in this study that showed measurable tritium greater than 0.2 tritium units and carbon-14 less than 75 percent modern carbon, indicative of a mixture of young and old groundwater, likely would benefit from additional investigation to determine the relative susceptibility to contamination.
[NOTE: The NWWA (now the NGWA) conducted comprehensive foundation work on susceptibility issues for EPA during the 1970s-1980s, resulting in the publication of the following publication: https://i2mconsultants.com/downloads/corp/20007KU4.pdf
The author and the USGS should have acknowledged this groundbreaking work in groundwater research.]
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