5.1 Study Area
5.2 Vadose Zone Leaching and Saturated Zone Mixing
5.3 Groundwater Flow
The objectives of this field application are to (1) identify nitrate contaminant leaching in the vadose zone, (2) determine mixing interaction in the saturated zone, (3) model groundwater flow, and (4) compare these results with historical data.
The study site is located at Wood River Junction in southern Rhode Island (Figure 5.1). Liquid wastes containing radionuclides and chemical solutes from an enriched uranium cold-scrap recovery plant have leaked from the plant site into the highly permeable, gravel aquifer. Nitrate concentrations and water tables of this area have been measured by U.S. Geological Survey on 1983 (Ryan et al., 1984).
This area is underlain by the Hope Valley Alaskite Gneiss, a metamorphic rock unit of Late Proterozoic age which is 570 to 900 million years old. The gneiss is an igneous rock unit that underwent one and possibly two episodes of metamorphism (Moore, 1959). The bedrock crops out east, northeast, west, and southwest of the study area, and unconsolidated glacial deposits of Pleistocene age have been deposited on top of the bedrock.
Glacial till deposits (poorly sorted clays, silts, sands, gravels, and boulders) form a relatively thin (less than 20 ft) mantle over the bedrock (LaSala and Hahn, 1960) and appear at the surface east of the plant site (Figure 5.1). In the bedrock valley, the glacial outwash deposits consist of predominantly medium to coarse sands and gravels in the first layer and mostly fine sands and silts in the second layer as shown in Figure 5.2. The porosity of these deposits is about 0.38. In this region, hydraulic conductivities of first and second layers were estimated that ranged from 140 to 190 [ft/d] and from 1.95 to 2.65 [ft/d], respectively by Ryan and Kipp (1984).
Figure 5.2 represents a cross section through a regional watershed bounded on one side by a topographic high, which marks a regional groundwater divide, and on the other side by the Pawcatuck river, which is a groundwater discharge area. This aquifer is assumed to be isotropic. The aquifer consists of water permeable porous material sitting on an impermeable bedrock layer.
The left and right groundwater boundaries assume groundwater divides and thus are impermeable, no-flow boundaries. Although no physical barrier exists, a groundwater divide has the same effect as an impermeable barrier because no groundwater crosses it.
The lower boundary is also a no-flow boundary because the impermeable bedrock forms a physical barrier to the flow. The upper boundary of the mathematical model is the horizontal line at the level of Pawcatuck river water table even though the water table of the physical system lies above the level of Pawcatuck river water table. Thus the effect of the aquifer at the level of Pawcatuck river water table is simulated through the measured water table values which are regarded as the potential values at the level of Pawcatuck river water table.
The ability of the first (vadose zone leaching) and second (saturated zone mixing) parts of the VG model to simulate soil columns in the source area, assuming homogeneity, is demonstrated in the profile of a single column presented in Figure 5.3.
The first (vadose zone leaching) and second (saturated zone mixing) parts of the VG model for this system involving a glacial outwash (porosity of 0.38) vadose zone and saturated zone environment in the source area were evaluated using a single soil column as listed in Table 5.1.
Input parameters of the time intervals at vadose zone concentration profile (PRTIME), groundwater impact and mass balance results (PTIME), and plot output (PLTIME) were typed 0.001 years (0.37 days). The horizontal width (WIDTH) and length (LENGTH) of the soil column were set 80 m (264 ft) by 80 m (264 ft) which is the same area of the source area.
The values of the recharge rate (RECHRG), the bulk density (RHOB), the effective porosity (POR), and the volumetric water content (THETA) have been used from the historical data (Ryan and Kipp, 1984). The value of the longitudinal dispersivity (ALDISP) has been used from the result value of the sensitivity analysis in section 4.5. The value of the top boundary condition (VALTBC) was the maximum value (580 mg/l) of the observation data by Ryan and Kipp (1984). The values of the other parameters were zero or the minimum values of the references.
The results of the vadose zone leaching simulation for the source area have been plotted as depth (ft) versus nitrate concentration (mg/l) for fourteen complete sets as shown in Figure 5.4. After six days of contamination in this area, nitrate concentrations of 250 (mg/l) should have reached groundwater table. Historical observation data (Ryan and Kipp, 1984) is the same as this simulation data. About 7.5 days later the maximum nitrate concentration loading at the bottom of vadose is 640 (mg/l) as shown in Figure 5.5. After 8 days the nitrate concentration will stay 580 (mg/l). Figure 5.6 presents the nitrate concentration of the saturated mixing zone. The concentration six days later is 60 (mg/l) which is close to the historical measurements by Ryan and Kipp (1984).
This part of the study involves a glacial pond and gravel aquifer with a contamination source approximately 900 meters from a river boundary in which the contaminant discharges. Glacial outwash sediments considered in this study have a mean value (K1 = 165 ft/day) for the first layer (sand and gravel) hydraulic conductivity (140 to 190 ft/day) and by adapting mean value (K2 = 2.3 ft/day) of second layer (fine sand and silt) hydraulic conductivity (1.95 to 2.65 ft/day). Those hydraulic conductivity values determined from lithologic logs and from analyses of three aquifer tests made within a 1-mi radius of the site (Ryan and Kipp, 1984).
The groundwater flow sub-model of the VG model is required for this heterogeneous (two layer) site. The upper boundary is located at y = 0 ft on the level of Pawcatuck River water table in 1983 for x distance ranging from 0 to 900 m. The distribution of head along this boundary was determined at 10 m intervals based on field measurements (Ryan and et al., 1985) from the topographic high to the level of Pawcatuck river water table (Table 5.2). The other three boundary conditions are for no-flow boundaries. The full set of boundary conditions for this study is written as follows.
Top hw(x,0) in Table 5.2 0<x<900 m
Three computer programs were applied to the field-measured water table data in Table 5.2. The grid intervals, Dx and Dy, are set to 10 and 1 m, respectively. As shown in Figure 5.7, the calculated potential head values and measured head values (Ryan and Kipp, 1984) are superimposed on the same figure. The programs closely produced head value contours (Figure 5.7) and flow lines intercepting at right angles (Figure 5.8).
The calculated head values and flow function values are plotted as contours in Figure 5.8. Plots of flow lines and equipotential lines are called flow-nets (Figure 5.8). Flow-nets are useful in depicting groundwater flow paths and calculating flux through the system. Two adjacent flow lines form a stream tube; for steady-state flow, the flux through the stream tube is constant. In a homogeneous and isotropic medium, flow lines and equipotential lines intersect at right angles and form curvilinear squares. The flow-net and observed nitrate concentration profile in May 1983 (Ryan and Kipp, 1984) are superimposed on the same figure as shown in Figure 5.8.
Figure 5.9 presents the calculation method of flow rate and travel time between 800 and 900 (m) from the source area. The flow rate and travel time calculated every 100 (m) were 3.05, 5.56, 5.56, 4.01, 1.29, 1.35, 2.19, 2.66, and 0.77 (m/day) and 33, 18, 18, 25, 77, 74, 46, 38, and 129 (days) from the source area (Figure 5.10). The total travel time of contamination was estimated to be 458 days.
Last modified: Oct 15, 1999
VG Model / Samuel Lee / VADOSE.NET