Groundwater flow

Such models are known as reactive transport models and are the subject of the next chapter (Chapter 21). We treat the preliminaries in this chapter, introducing the subjects of groundwater flow and mass transport, how flow and transport are described mathematically, and how transport can be modeled in a quantitative sense. We formalize our discussion for the most part in two dimensions, keeping in mind the equations we use can be simplified quickly to account for transport in one dimension, or generalized to three dimensions. 

Transport in flowing groundwater is controlled primarily by the pattern and rate of flow, which are described by Darcy s law. Darcy s law says that groundwater migrates from high hydraulic potential to low, according to, 

Hydraulic potential is the sum of the VdP work done on the water and its potential energy. The quantity is given by, 

Darcy s law can be written in an alternative form in terms of hydraulic head h (cm), the height to which water would rise above sea level if free to flow into a well, and hydraulic conductivity Kx, Ky (cm s-1) of the sediment. In this case, groundwater flows from high head to low, at a discharge given by, 

Changes in hydraulic head reflect variation in the hydraulic potential, according to d t = pg dh, and hydraulic conductivity is proportional to permeability, 

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Locational considerations include both surficial location and screened interval, ie, the sampling depth. The surficial location is selected based on whether the sample is to represent background quaUty or quaUty at the location of contamination, or potential leak location. In selecting the surficial location, the groundwater flow parameters, velocity and direction, are assumed to be known from other monitoring wells or borings already completed. 

Predict major boundaries, avenues of groundwater flow... 

Subsurface drains are essentially permeable barriers designed to intercept the groundwater flow. The water must be collected at a low point and pumped or drained by gravity to the treatment system (Figure 8). Subsurface drains can also be used to isolate a waste disposal area by intercepting the flow of uncontaminated groundwater before it enters into a contaminated site. 

As indicated earlier, heavy contamination can be buried, sealed or removed. Burying of the material should be well below the root growth zone, and this is normally taken as 3.0 m below the final ground-surface level. Sealing for heavy contamination to prevent vertical or lateral leaching through groundwater flow can be with compacted clay or proprietary plastic membranes. Removal from site of the contaminants is normally only contemplated in a landscaped scheme where the material, even at depth, could be a hazard to public health directly or phytotoxic to plant life. 

The excess of evaporation from the oceans is made up for by runoff from the land. Although this flux is much smaller than precipitation and ET, it is a major link in many cycles and is of particular importance to humans in terms of water supply. Runoff can be broadly categorized into subsurface, or groundwater, flow and surface flow, consisting of overland runoff and river discharge. 

Figure 6-7 illustrates the runoff paths for HOF and SOF, as well as for subsurface stormflow and groundwater flow. Subsurface stormflow is a moderately rapid runoff process in which water flows to a stream through highly permeable surface soil layers (without reaching the water table). Note in Fig. 6-7 that while HOF and subsurface stormflow may occur over a large fraction of an infiltration-limited hillslope, SOF occurs over a smaller portion adjacent to the stream. 

Fig. 6-7 Vertical cross-section showing pathways for surface and subsurface runoff. Path 1 HOF path 2 groundwater flow path 3 subsurface stormflow path 4 SOF. (From Dunne and Leopold, 1978.)...

Fig. 8-7 Three principal ratios control the style of runoff generation prevalent in a landscape (1) ratio of rainfall intensity to the infiltration capacity of the soil (2) ratio of bedrock conductivity to soil conductivity and (3) the topographic index defined by the ratio of the upslope drainage area to the ground slope. HOF = Horton overland flow SOF = saturation overland flow SSS = subsurface stormflow GWR = groundwater flow.

Pumping during well development performs two important functions. First, pumping removes the materials from the borehole left behind by drilling. Second, as the water in the well is removed, groundwater flow velocity from the surrounding formation increases when it reaches the higher permeability filter pack around the well... 

Processes controlling nuclide distributions. The general equations for onedimensional advective transport along a groundwater flow path of groundwater constituents, and the incorporation of water/rock interactions, are given in such texts as Freeze and Cherry (1979). The equations can be applied to the distribution in groundwater of each isotope I with a molar concentration Iw and parent with Pw to obtain... 

Iw with distance x along a groundwater flow path. The terms on the right side represent the input and removal terms (see Fig. 2a), which in order are ... 

The migration rate of a groundwater constituent, relative to the groundwater flow rate, is controlled by the retardation factor, where Ri = 1 + Ki. Where Ki 1 (e.g., for Th and Ra), Ri Ki, and Iads + Iw = IwRi- Note that ki and k-i are element-specific but not isotope-specific. All isotopes that decay slower than desorption, so that k-i have a value of Ki that is equal to that of a stable isotope (Eqn. 3). The value of Ki may be lower for the shortest-lived nuclides (see Fig. 2b), and so a series of equations derived from Equation (3) applied to different isotopes of the same element may be used to obtain absolute values for the separate rate constants. 

Corbett, DR, Dillon K, Burnett W (2000a) Tracing groundwater flow on a barrier island in the northeast Gulf of Mexico. Estuar Coast Shelf Sci 51 227-242... 

Luo SD, Ku XL, Roback R, Murrell M, McLing XL (2000) In-situ radionuclide transport and preferential groundwater flows at INELL (Idaho) decay-series disequilibrium studies. Geochim Cosmochim Acta 64 867-881... 

Suksi J, Rasilainen K, Casanova J Ruskeeniemi T, Blomqvist R, Smellie JAT (2001) U-series disequilibria in a groundwater flow route as an indicator of uranium migration processes. J Contam Hydrol 47 187-196... 

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