Site hydrology

Remediation is a process of restoration. In order to conduct remedial activities, we must first understand both the process which generated the contaminants, the contaminant chemistry, the soil geology, the site hydrology, and the nature and kind of interactions between the soil, water, and the contaminants. In this brief exposition, weTl discuss a couple of different types of sites and see the limits of remediation technology. 

Drainage basin boundary Sampling site hydrologic cataloging unit based on samples collected at gaging stations located near the terminus of the basin. 

The preferred on-site hydrology method is the rational method, requiring a minimum concentration time of 10 minutes. Often the concentration time for the contributing on-site pavement runoff is less than 10 minutes. The initial concentration time is determined by an overland flow method until the runoff is concentrated in a curbed section. Channel flow using the roadway-curb cross-section should be used to determine velocity and flow time to the first inlet. The channel flow velocity and flooded width are calculated by Manning s formula ... 

The ICRCL trigger values are intended to be maximum total concentrations for a particular land use that do not increase risk to selected targets (Simms Beckett 1987). However, the potential toxicity to specified targets (such as humans, plants, etc.) will depend very much on the form in which the metal exists (Solomon Powrie 1992), as well as the site hydrology. While comparison of total contaminant levels... 

From the previous review, it is apparent that many factors must be considered when evaluating the mobility of toxic metals from contaminated industrial sites. Consequently, any investigation must determine the total level of each contaminant, identify its chemical specia-tion and its likely leaching rate. An indication of current leaching rate can be obtained from the site hydrology and metal levels in site waters. Establishing solid phase chemical speciation requires the synthesis of data from direct tech-... 

Depression method. At locations where natural or artificial depressions exist, it is often possible to use them effectively for land-filling operations. Canyons, ravines, dty borrow pits, and quarries have oeen used for this purpose. The techniques to place and compact solid wastes in depression landfills vary with the geometiy of the site, the characteristics of the cover material, the hydrology and geology of the site, and access of the site. 

Solid-wa.ste-filling plan. The specific method of filling will depend on the characteristics of the site, such as the amount of available cover material, the topography, and local hydrology and geology. To assess future development plans, it will be necessary to prepare a detailed plan for the layout of the individual solid-waste cells. On the basis of the characteristics of the site or the method of operation (e.g., gas recovery), it may be necessaiy to incorporate special features for the control of the movement of gases and leachate from the landfill. 

Proximity to critical areas specified in government regulations, accessihiUty, site geology and hydrology. 

Determine interception points Existing site data, hydrologic for withdrawal options, areas literature site inspection of capping... 

Sensitive sites would include sites that are receiving high inputs of atmospheric Hg deposition and sites with aquatic ecosystems where top end predators have high levels of Hg. We also recommend that urban sites with elevated atmospheric Hg deposition and forest sites with shallow hydrologic flowpaths, wetlands, and unproductive aquatic ecosystems should strongly be considered as candidate sites. 

Schirmer, M., Butler, B., Barker, J., Church, C., and Schirmer, K., Evaluation of biodegradation and dispersion as natural attenuation processes of MTBE and benzene at the Borden field site, Physics and Chemistry of the Earth Part B Hydrology, Oceans and Atmosphere, 24 (6), 557-560, 1999. 

The ET cover cannot be tested at every landfill site so it is necessary to extrapolate the results from sites of known performance to specific landfill sites. The factors that affect the hydrologic design of ET covers encompass several scientific disciplines and there are numerous interactions between factors. As a consequence, a comprehensive computer model is needed to evaluate the ET cover for a site.48 The model should effectively incorporate soil, plant, and climate variables, and include their interactions and the resultant effect on hydrology and water balance. An important function of the model is to simulate the variability of performance in response to climate variability and to evaluate cover response to extreme events. Because the expected life of the cover is decades, possibly centuries, the model should be capable of estimating long-term performance. In addition to a complete water balance, the model should be capable of estimating long-term plant biomass production, need for fertilizer, wind and water erosion, and possible loss of primary plant nutrients from the ecosystem. 

Surface runoff (Q) is the second-largest part of the hydrologic water balance for ET landfill covers at many sites in humid regions. Even at dry sites where surface runoff is small, errors in estimates of Q are important, and especially so if the model estimates significant Q on days with no runoff. Estimates of Q are therefore important to the design process at all sites. 

Some models require calibration to optimize input parameters they are best used in a research setting where it is possible to make measurements with which to calibrate the model for a particular site. Appropriate measured hydrologic data are seldom available to calibrate a model for a particular landfill site. Therefore, engineering models used for ET cover design should not require calibration. 

Schroeder, P.R. and Gibson, A.C., Supporting Documentation for the Hydrologic Simulation Model for Estimating Percolation at Solid Waste Disposal Sites (HSSWDS), Draft Report, U.S. Environmental Protection Agency, Cincinnati, OH, 1982. 

At this point it is important to note that the flow model (a hydrologic cycle model) can be absent from the overall model. In this case the user has to input to the solute module [i.e., equation (1)] the temporal (t) and spatial (x,y,z) resolution of both the flow (i.e., soil moisture) velocity (v) and the soil moisture content (0) of the soil matrix. This approach is employed by Enfield et al. (12) and other researchers. If the flow (moisture) module is not absent from the model formulation (e.g., 14). then the users are concerned with input parameters, that may be frequently difficult to obtain. The approach to be undertaken depends on site specificity and available monitoring data. 

Split-sample" field testing Involving calibration and verification on separate data sets, often for different time periods at one site. This approach has been used widely in hydrologic modeling and has general utility for model testing. 

---