Soil cores were collected through the unsaturated zone at two sites in the Powder River Basin near Gillette, Wyoming. The spreadsheet problem7.1.xls shows depth profiles of concentrations (migrogram per gram of sediment) of chloride and nitrate (as N) and gravimetric soil-water content (gram of water per gram of sediment).

A) Assuming that depositional rates have not changed over time and that dry deposition is equivalent to wet deposition, estimate the rate of drainage through the unsaturated zone at the two sites by applying the chloride mass balance method. Requisite data can be obtained from the ( National Climatic Data Center ) and the National Atmospheric Deposition Program. Site 1 has a distinctive bulge in both the chloride and nitrate profiles. Estimate drainage on the basis of concentrations beneath the bulge and within the bulge.

B) Calculate the total mass of both chloride and nitrate within the Site 1 profile. Under present deposition rates, how long do you estimate that it has taken for the chloride and nitrate to accumulate? Assume the bulk density of the sediment is 1.65 g/cc.

C) A nitrate mass balance approach can also be used to estimate drainage rates. However, there can be sources for unsaturated-zone nitrate other than atmospheric deposition, and there can be sinks for nitrate as well. With these considerations in mind, apply the nitrate mass balance method to estimate drainage rates. Discuss possible sources and sinks for nitrate in the unsaturated zone.

D) The water table is at depths of 25.00 m at Site 1 and 28.65 m at Site 2. Chloride concentrations in groundwater are 21.2 mg/L (Site 1) and 5.6 mg/L (Site 2). Apply the groundwater chloride mass balance method to estimate recharge at the two sites. Compare and discuss all of the different estimates of drainage and recharge for the two sites. The two sites are located at within 0.5 km of each other with similar elevation, slope, and aspect. What could explain the differences in chloride and nitrate profiles and recharge/drainage estimates between the two sites?

Problem 7.2

Twenty two wells in unconfined aquifers in central Nebraska were sampled for CFC concentrations. The data are contained in the spreadsheet problem7.2.xls. Calculate apparent ages for each sample for all 3 CFC values. Identify contaminated samples. How do the ages for the different CFCs compare? How sensitive are results to elevation, recharge temperature, and excess air? Plot age as a function of depth below the water table. Can these ages be used to estimate recharge?

CFC data are given in pg/kg water. Convert to pmol/kg. Mixing ratios, x, can then be determined as:

x = C/((K_h*(P - P_vapor) + EA/22414.1)

where C is measured concentration in pmol/kg;

K_h is Henry's Law constant and is determined on the basis of recharge temperature, T, according to the formula given at the USGS CFC Laboratory web site;

P is total atmospheric pressure in atmospheres and can be determined as P = exp(-E/8300), where E is elevation in m;

P_vapor is atmospheric water vapor pressure. If 100% relative humidity is assumed P_vapor can be estimated on the basis of recharge temperature, T, in Kelvin: P_vapor = exp(24.4543 - 6745.09/T - 4.8489*ln(T/100));

EA is excess air in ccSTP/kg water; and 22414.1 is the conversion factor for pmol/kg.

Once the mixing ratios have been determined, apparent recharge dates can be obtained from Figure 7.2 (more accuracy can be obtained by referring to table in Appendix II of Use of Chlorofluorocarbons in Hydrology - A Guidebook, International Atomic Energy Agency (2006). Apparent ages are the difference between sampling and recharge dates.

Problem 7.3

Water samples were obtained from two monitoring wells for dating by the tritium helium-3 method. The wells were located within 10 m of each other and tapped an unconfined sand aquifer. Well screens were 1-m long. Well A was screened at a depth of 13-14 m below land surface; Well B was screened at a depth of 19-20 m below land surface. With the following data, calculate apparent ages for the two samples taking into account excess air and terrigenic production of helium. Assume aquifer porosity is 0.35 and that total aquifer thickness is 25 m. How important are excess air and terrigenic sources of helium? Explain how the calculated ages might be used to estimate recharge.

Relevant equations can be found at the USGS CFC Laboratory web site. Use the following equations for neon and helium concentrations in equilibrium with air at the recharge temperature:

C_eq = exp(a1+a2*(100/T)+a3*(ln(T/100))+a4*(T/100))/(1000*exp(E/8350))

where T is in Kelvin and elevation, E, is in m.

a1 = -167.2178 (He) -170.6018 (Ne)

a2 = 216.3442 (He) 225.1946 (Ne)

a3 = 139.2032 (He) 140.8863 (Ne)

a4 = - 22.6202 (He) - 22.629 (Ne)

and C_eq is equilibrium concentration in ccSTP/g water

Elevation of land surface is 122.13 m. Estimated recharge temperature is 13.5 degrees Celsius.

Date of sampling July 8, 2007

Depth to groundwater below land surface: 7.55 m (Well A), 8.21 m (Well B)

Measured tritium concentration (TU): 5.67 TU (Well A), 4.43 TU (Well B)

Measured ⁴He concentration (ccSTP/g water): 6.885e-8 (Well A), 7.491e-8 (Well B)

Measured ³He/⁴He ratio: 1.5111e-6 (Well A), 1.6726e-6 (Well B)

Measured Ne concentration (ccSTP/g water): 2.5593e-7 (Well A), 2.7422e-7 (Well B)

Assume that the ratio of terrigenic (³He/⁴He) is 2.e-8 and that 1 TU = 4.018694e14 ccSTP/g water.

Contact Information