Groundwater flow rates calculation

The residual inventory, or excess , is then divided by the concentration of the tracer in the discharging groundwater to calculate the groundwater flow rate. 

Fig. 6. Equilibration distances calculated for albite at different groundwater flow rates and fracture apertures in a hypothetical parallel plate fracture. Distances were calculated at 25°C and neutral pH in pure water using a transition-state theory rate equation and experimental data from Knauss Wolery (1986).

Figure 21.2 shows how in the calculation results benzene is transported through the aquifer. The pulse of benzene migrates at the rate of groundwater flow, traversing the aquifer in ten years. As a result of biodegradation by the natural microbial consortium, however, the benzene concentration decreases markedly with time, compared to the non-reacting case. 

Groundwater-inflow rates as calculated by the solute and isotope mass-balance methods for several northern Wisconsin lakes are listed in Table I. Dissolved calcium was used as the solute tracer because it is the constituent whose concentration differs the most between groundwater and precipitation, the two input components to be separated by the method. In addition, calcium is nearly conservative in the soft-water, moderately acidic to cir-cum-neutral lakes in northern Wisconsin. Results from the two methods agree relatively well, except for Crystal Lake, where groundwater-flow reversals are frequent. 

The rainwater of Bermuda is in near equilibrium with atmospheric Pc02 = 10-3.5 atm., and contains small amounts of sea salt (0.07 wt. % seawater). The rainfall of 147 cm y1 is seasonally distributed. The rain enters the saturated zone by two main paths direct rainfall on marshes and ponds, and percolation downward from the vadose zone as vadose seepage and flow through rocks during times of soil water excess (Vacher, 1978). Total annual recharge of the saturated zone is about 40 cm y-1 (Vacher and Ayers, 1980). The residence time of the groundwater has been calculated as 6.5 years, and the average age of the sampled water as 4 years (Vacher et al., 1989). Such estimates are necessary for calculations of carbonate mineral stabilization rates, as shown in a later section. 

As stated in section 2.10, the velocity by which groundwater flows is commonly calculated from the water table gradient and the coefficient of permeability (k, or the related parameter of transmissivity). The k value is determined by a pumping test. During such a test a studied well is intensively pumped and the water table is monitored in it as well as in available adjacent observation wells. The change in water table level as a function of the pumping rate serves to compute the aquifer permeability. 

Example 8-2 (taken from Bolton et al, 2001a) A contaminated groundwater that contains 200 pg of trichloroethene (C2HCI3) is treated with a 25 kW flowthrough AOT at a flow rate V of 20 m h . The effluent concentration of C2HCI3 had dropped to 5 pg Calculate the appropriate design parameter. 

The actual calculations were performed using balance, an earlier inverse mass balance modeling code that is now superseded by netpath and phreeqc. Chapelle and Lovley (1990) then calculated the time interval in these segments from flow velocities calculated by a numerical groundwater flow model and the length of the flow path. The total CO2 production rate from oxidation of organic matter is... 

The calculated flow paths near the transect in Nieschen show an increasing influence of the groundwater level raising up towards the ditch floor (Fig. 8.4). Near the eastern edge of the ditch, the influence decreases rapidly. This exfUtration zone is dominated by intensive mixing effects of deep and shallow groundwater. The total exfiltration rate, calculated for 1999, reached 202 m /m. ... 

Validation of the hydraulic model was achieved using a steady-state solution of the groundwater flow and exfiltrating rate calculated by the groundwater modelling system FEFLOW. These calculations are carried out at a transect in Nieschen (see Section 8.3.2.3). Detailed geologic and hydraulic data and long term measurements enables an exact parameterization of the exemplary ditch types. 

The reactive transport of contaminants in FePRBs has been modeled using several approaches [179,184,186,205-208]. The simplest approach treats the FePRB as an ideal plug-flow reactor (PFR), which is a steady-state flow reactor in which mixing (i.e., dispersion) and sorption are negligible. Removal rates (and therefore required wall widths, W) can be estimated based on first-order contaminant degradation and residence times calculated from the average linear groundwater velocity [Eq. (27)]. The usefulness of... 

Tritium measurements are frequently used to calculate recharge rates, rates or directions of subsurface flow, and residence times. For these purposes, the seasonal, yearly, and spatial variations in the tritium content of precipitation must be accurately assessed. This is difficult to do because of the limited data available, especially before the 1960s. For a careful discussion of how to calculate the input concentration at a specific location, see Michel (1989) and Plummer et al. (1993). Several different approaches (e.g., piston-flow, reservoir, compartment, and advective-dispersive models) to modeling tritium concentrations in groundwater are discussed by Plummer et al. (1993). The narrower topic of using environmental isotopes to determine residence time is discussed briefly below. 

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