Ground Water Recharge Using Waters of Impaired Quality (1994) / Chapter Skim
Currently Skimming:
3 Soil and Aquifer Processes
3 Soil and Aquifer Processes
Pages 97-131
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 97... ...
Thus this chapter reviews the principal processes governing transport and fate in soil first in a generic fashion, and later with respect to the behavior of specific chemical or pathogen groups. The soil properties that are important in a properly functioning soil-aquifer treatment (SAT)
|
From page 98... ...
Soil-aquifer processes can be counted on to provide treatment benefits. Soil Properties The ideal porous medium for an SAT operation is one Hat allows rapid infiltration and complete removal of all constituents of concern.
|
From page 99... ...
If clays disperse, infiltration rates drop. Chemical reactions between the recharge water and the native ground water can cause precipitates to form and these can clog pores and reduce injection capac
|
From page 100... ...
Thus, the role of the unsaturated or vadose zone in recharge systems is to help filter out or transform harmful constituents in the soil solution as recharge water moves through the soil matrix en route to the aquifer. The vadose zone is a much more complex transport medium than an aquifer, for several reasons.
|
From page 101... ...
Aquifer Processes Chemicals or pathogens that are still present in solution when the recharge water reaches the aquifer are subject to many of the same processes that occur in the vadose zone, with several exceptions. The biological activity in ground water is much slower than in the near-surface zone, so degradation is greatly reduced.
|
From page 102... ...
In addition, the soil can offer substantial resistance to the transport of chemical from the soil profile to the surface, particularly if the soil is wet and little upward flow of water is occurring. Therefore, in a typical SAT process where recharge water is ponded over the surface for prolonged periods of time, the primary route for volatilization loss will be from the surface of the standing water.
|
From page 103... ...
Clearly, compounds with half lives Hat are considerably less than the detention time of the water on the surface will not enter the soil. For instance, volatilization losses of 22 to 73 percent were found for a wide spectrum of hydrocarbons in sewage effluent infiltration basins in Phoenix, Arizona (Bouwer et al., 1986~.
|
From page 104... ...
However, the presence of standing water on the surface enhances the loss from water, so that any chemicals with volatilization half-lives that are significantly less than the total detention time in surface water will probably evaporate out of solution before they ever reach the soil. Recharge water containing fertilizer or sewage contributions may contain nitrogen compounds that can be transformed to volatile species.
|
From page 105... ...
However, in the process of averaging, some of the solute motion is lost. The small-scale migration of solution Trough tortuous pathways within the porous medium no longer appears in the water flux expression after volume averaging, because the fluctuations about the mean motion do not contribute to the net water transport.
|
From page 106... ...
Solute Sorption During Transport Chemicals that are hydrophobic or positively charged do not travel at the speed of the flowing water, but rather are slowed by their attraction to stationary solid sorption sites. Although the sorption process is very dynamic at the molecular scale, it is useful to conceptualize an adsorbed molecule as existing in a distinct phase that is temporarily immobilized by virtue of its attachment to stationary solid matter.
|
From page 107... ...
where for is the fraction of soil organic carbon content (mass per mass of soil) and Koc is the distribution coefficient per unit organic carbon, called the organic carbon partition coefficient.
|
From page 108... ...
They found nitrate fronts moving very slowly underneath these fields and estimated that as much as 50 years would be required for the chemicals to reach deep ground water aquifers under the fields. Pesticides or other contaminants that also adsorb to soil solids would have considerably longer travel times under the same conditions.
|
From page 109... ...
Colloidal complexes formed win mineral or organic matter suspended in solution are subject to He same filtration action. IMPORTANT SOIL-AQUIFER PROPERTIES Vadose Zone Properties The effect of the soil properties in ground water recharge depends on the method of recharge used.
|
From page 110... ...
The vadose zone must function as a water purification system as well as a conduit to the aquifer. Depending on the operation and the constituents of the recharge water and soil, the following physical, chemical, or biological transformations of contaminants within the water may occur: filtration of suspended solids, parasites, and bacteria; sorption of trace elements, bacteria, and viruses; precipitation of phosphates and trace metals; biodegradation of organics; recarbonation of high pH effluents; and denitrification (Asano, 1985~.
|
From page 111... ...
In addition, in coastal areas and in certain arid regions, recharge waters may be cycled through aquifers containing waters of poor quality, and here it is also desirable to segregate the recharge and native waters so as not lo impair Me quality of the recharge water prior to extraction. To a large extent, the detention time of recharge water during subsurface flow determines the improvement in water quality.
|
From page 112... ...
Spatial variability is primarily important where the recharge water is very clean and infiltration rates are controlled by soil hydraulic conductivity. Where
|
From page 113... ...
This phenomenon has a number of possible causes, including density and viscosity differences between the antecedent and incoming water, air entrapment during infiltration, and downward flow from a fine-textured to a coarse-textured soil layer (Hillel, 1987~. Preferential flow has serious implications for chemical
|
From page 114... ...
TRANSPORT AND FATE OF SPECIFIC CONSTITUENTS OF RECHARGE WATER The principles discussed in the previous sections are useful to understanding the chemical and pathogen removal processes that occur during transport through soil and aquifer material. This section looks at categories of solution constituents that correspond to groupings used elsewhere in this report.
|
From page 115... ...
The primary removal mechanism in these operations is denitrification, with perhaps some ammonia volatilization losses during drying cycles. Phosphorus Phosphates in recharge water are removed by precipitation to amorphous or crystalline forms with iron, aluminum, or calcium.
|
From page 116... ...
Positively charged trace metal ions can be attenuated by ion exchange with negatively charged clay mineral surfaces. This process is a partition reaction affecting all cations in soil and therefore is not a significant removal mechanism for trace elements at low values.
|
From page 117... ...
Low organic carbon levels generally limit microbial action in deeper regions of the vadose zone and in aquifers. As discussed in a previous section, the travel time of an organic chemical may be roughly designated by its retardation factor in a given soil, or more intrinsically by its organic carbon partition coefficient Koc.
|
From page 118... ...
Field measurements of viral movement at ground water recharge sites are limited. At the Flushing Meadows surface spreading site in Phoenix, Arizona, viruses were not detected after 10 to 30 m (33 to 98 ft)
|
From page 119... ...
Nature of Reduced Carbon Species A major uncertainty arises from the fact that most of the product identification work has been done on standardized humic or fulvic acid fractions isolated from surface water sources of sufficient quality (low organic carbon levels) to be considered as a traditional water supply source.
|
From page 120... ...
The effect of such changes on soil retention, metal binding capacity, interactions with other organics, and so on, is unknown. However, one might speculate that this partially oxidized carbonaceous material might be less retentive on soil organic matter because of decreased hydrophobicity and more retentive on soil mineral phases because of increased charge density.
|
From page 121... ...
Halogenated organic compounds, including the nonvolatile DBPs, will have greater hydrophilicity and correspondingly less retention on soil organic matter and more rapid transport to the water table. Some DBPs, such as the halogenated acetic acids, will behave as fully ionized organic anions at the pH values of water in the vadose zone and may be expected to move at the same speed as the subsurface water.
|
From page 122... ...
SUSTAINABILITY OF THE SAT SYSTEM Sustainability refers to the long-term viability of an SAT operation, and specifically to the ability of the soil and aquifer to receive recharge water indefinitely without suffering deleterious side effects. Within the vadose zone, clogging is an inevitable side effect of SAT procedures, but is largely manageable with periodic drying to oxidize the accumulated organic material and restore the infiltration rates, along with periodic physical removal of the clogging layer by scraping, raking, or other techniques (Bouwer, 1985~.
|
From page 123... ...
SOIL AND AQUIFER PROCESSES P1D INR if.
|
From page 124... ...
Pesticides and other organic chemicals present in recharge water vary significantly in their mobility, persistence, and suspected health effects. Water that contains pesticides known to be resistant to microbial or chemical degradation in soil may pose problems for SAT systems in the long run, because refractory organics are only slowed by solid-phase sorption and not pennanendy removed from solution.
|
From page 125... ...
Tracing of recharge waters is more difficult if the recharge water quality is similar to that of native formation waters. At the Fred Hervey water reclamation
|
From page 126... ...
For example, chloride and sodium concentrations were used to determine mixing of reclaimed water recharged at East Meadow, Long Island, with the ambient ground water (Schneider et al., 1987~. The median sodium and chloride concentrations in the recharge water were approximately 115 and 157 mg/1, respectively, whereas those in ambient ground water were 22 and 27 mg/1, respectively.
|
From page 127... ...
For example, recharge waters in El Paso, Texas, are planned for potable reuse. The moderately weak domestic sewage receives advanced wastewater treatment before injection, and the recovered ground water receives only disinfection before use.
|
From page 128... ...
Organic chemicals are removed to varying extents by volatilization or chemical or biological degradation during passage through the vadose zone. Some pathogen removal in soil occurs by filtration in the surface clogging mat for the largest organisms and by sorption for bacteria and viruses.
|
From page 129... ...
1985. Soil deposition of trace metals during groundwater recharge using surface spreading.
|
From page 130... ...
1990. Preferential flow in a sandy vadose zone: 1.
|
From page 131... ...
1981. The Thel~odynamics of Soil Solutions.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.