|Ground Water – Surface Water Exchange
A View from Inside the Unconfined Karstified Floridan Aquifer with Implications for Ground Water Protection
Because of its high volatility, 222Rn gas quickly dissipates when exposed to the atmosphere creating a significant disequilibria between concentrations in ground water and surface water (Rogers, 1958). 222Rn concentrations are measured in alpha-scintillation counters and reported in Becquerels per liter (Bq/L) which equate to the number of alpha particle disintegrations per second per liter.
Rogers (1958) demonstrated that elevated 222Rn concentrations measured in streams and rivers are indicative of ground water inputs. Furthermore, Ellins and others (1990, 1991) showed that by accounting for gas exchange across the air-water boundary, elevated measures of 222Rn can be used to quantify ground water influx to a stream or river. The same principles were applied in this investigation but in reverse. When sampling the Floridan aquifer from inside the Devil’s Ear cave system, low 222Rn concentrations indicated river water intrusion. The difference between the 222Rn concentration in a sample and that of a pure aquifer standard was used as the basis for quantifying river water intrusion.
Solving the equation for the percentage of river water in a given sample produces
Background 222Rn concentrations in the aquifer were measured by sampling five wells near the field area but more than 1 km from the river. Raq was determined to be 13.0 Bq/L in February 1992 and 14.2 Bq/L in June 1993 by averaging the values obtained from the five wells. Measuring Rriv in the field area revealed values ranging between 4.2 and 9.0 Bq/L. These values were not considered accurate estimations of the background 222Rn concentration in the river because of large ground water inputs from several springs. Instead, Rriv was determined by averaging several measurements collected by Ellins and others (1991) and Hisert (1994) that were taken just upstream of the field area where there is less ground water input. Averaging these reported 222Rn concentrations produced a value for Rriv of 1.0 Bq/L. The model only assumes fixed end-member concentrations for the mixing waters. Note that a greater value for Rriv would result in an increased value for %Riv.
Delta Oxygen-18 (d18O) Confirmation
Sampling the Devil’s Ear Cave System
222Rn sampling was conducted following the methodology described in Key (1981) and Ellins and others (1990). Water samples for 222Rn measurements were collected in evacuated 250 ml plastic bottles. The bottles were filled with approximately 150 ml of aquifer water leaving 100 ml of head space to collect the 222Rn gas that would be volatilized from the water sample. After surfacing, the samples were transported to a laboratory where the gasses in the head space were extracted from the sample bottles into Lucas-type counting cells. After sufficient time to allow the 222Rn to equilibrate with it’s daughter products, the cells were placed into alpha-scintillation counters where light photons emitted by alpha particle disintegrations were counted and recorded as Becquerels per second per liter of sample. Key (1981) reports that the error associated with this method does not exceed 14%.
d18O samples were collected in 50 ml glass vials, precleaned with nitric acid and filled with distilled water prior to the dive. The vials were flushed with air from the scuba cylinders at the underwater sampling locations then rinsed and refilled with aquifer water.
A more detailed accounting of the methodology used to collect samples for 222Rn and d18O as well as a description of the error involved with the data collection is provided in Kincaid (1994).