Studies to Estimate Groundwater Recharge
in the Central Carrizo Wilcox Aquifer
Scanlon, principal investigator; Robert C. Reedy
estimated using Cl concentrations in the unsaturated zone and groundwater
and using 3H/3He concentrations in groundwater in 7 boreholes in the
outcrop area of the Simsboro Formation in the central part of the
Carrizo Wilcox aquifer (Fig. 1). Long-term (50 yr) mean annual precipitation
in the central part of the Carrizo-Wilcox aquifer ranges from 29 inches
in the southwest to 48 inches in the northeast of the area.
concentrations in the unsaturated zone ranged from 23 to 519 mg/L.
Variability in mean chloride concentrations was high locally. Chloride
concentrations were also highly variable within each profile. There
was no systematic variation in chloride concentrations with depth.
Recharge rates were calculated for the portion of the profiles that
generally represented the last 50 yr where possible. In some cases
recharge rates were so low that a 50 yr section corresponded to a
very narrow depth interval. Recharge rates generally ranged from 0.2
to 1.4 in/yr. The time required to accumulate chloride in each profile
ranged from 110 to 2815 yr. Groundwater chloride concentrations were
generally lower than those in the unsaturated zone (5 - 180 mg/L).
Representative recharge rates based on groundwater chloride concentrations
range from 1 to 1.5 in/yr and are higher than those based on unsaturated
zone chloride. The generally higher recharge rates based on groundwater
chloride relative to unsaturated zone chloride are considered more
representative of the regional system whereas the unsaturated zone
data indicate that locally recharge rates are lower.
concentrations ranged from 0.76 to 3.57 TU.
These tritium levels were much greater than the detection limit for
tritium (~ 0.2 TU) and indicate that a component of the water was
recharged in the last 50 yr. Tritium/helium was also used to date
the water in 3 of the 7 wells (CW3, CW4, and CW6). There were problems
with analysis of 3He in water samples from CW6. 3He concentrations
were low in well CW3 and much higher in well CW4. The low 3He concentrations
in CW3 indicate a short residence time of the water of 2.2 - 6.1 yr
(replicate analyses) whereas the much higher 3He concentrations in
CW4 indicate a residence time of 21.4 yr. The times represent the
time of 3He accumulation since it was isolated from the unsaturated
zone. Water velocities were calculated by dividing the distance between
the water table and the center of the well screen by the age of the
water and resulted in velocities of 0.4 (CW4) to 1.4 - 4.0 ft/yr (CW3).
Recharge rates of 1.6 (CW4) to 6.0 - 16.7 in/yr (CW3) were calculated
by multiplying the velocities by the average porosity of 0.35. The
recharge rate for CW4 of 1.6 in/yr is similar to that estimated from
the groundwater chloride concentration. The recharge rate for CW3
of 6.0 - 16.7 in/yr is high similar to the high recharge rate calculated
the groundwater chloride concentration (6.2 in/yr). Future work should
consider using gas diffusion samplers for tritium-helium dating rather
than copper tubes used in this study, particularly in low yield wells.
This approach would help reduce the uncertainties in the age estimates.
|Reedy, R. C.,
Scanlon, B. R., and Dutton, A. R., 2003, Collection and analysis of
environmental tracers for estimation of recharge rates in the GAM
model of the central Carrizo-Wilcox aquifer, in A. R. Dutton,
B. Harden, J. P. Nicot, and D. O'Rourke, editors, Groundwater Availability
Model for the Central Part of theCarrizo-Wilcox Aquifer in Texas.
Contract Report prepared for the Texas Water Development Board.