Summary and Conclusions

The ultimate conclusion arising from the above discussion is that reliable contaminant monitoring in vertically heterogeneous systems requires a 3-D understanding of the system. Practical constraints in hydrology often mean that 3-D characterization and modeling are impossible, but even a qualitative appreciation for 3-D effects is useful. With this in mind, this report and the accompanying interactive 3-D model were developed to allow direct exploration of 3-D transport effects at one well-characterized site, Hays, Kansas. Decades of experience at Hays indicated that the Quaternary alluvial aquifer contained two principal facies, one (sand-channel facies or Qal) with substantially higher hydraulic conductivity. Given dense subsurface information about the distribution of these facies a 3-D geometric and hydrologic flow model was developed. Calibration of the flow model established the ratio of conductivities for Qal/Qt at 10/1. Transport modeling (particle tracking) using this flow model established that the distribution and connectivity of the high-conductivity facies (Qal) strongly controls water movement beneath Hays. A qualitative indicator of aquifer contamination susceptibility is the presence of unconfined sand facies (Qal) along well-connected zones of Qal. Comparison of the transport results to observations (existing PCE plume) shows remarkable agreement.

The model results and interactive display are highly instructive in illustrating when 2-D modeling is adequate in heterogeneous settings, and when 3-D evaluation is required. In general, effective drinking water aquifer management in the presence of known or suspected subterranean contaminants will require detailed and often 3-D characterization and modeling. For the defined purposes of WHPA delineation, which are to limit future aquifer contamination, simple 2-D models will often be sufficient for planning, even for multi-layer systems. When existing aquifer contamination and vertical heterogeneity are present, even if the contamination is not the focus of the study, a fully 3-D approach will generally be required to adequately describe the system. This exposes a dilemma faced by many localities: some aquifer degradation has already occurred, yet budget constraints allow only local or highly simplified analysis (i.e. WHPA delineation). The results of a typical WHPA delineation may help avoid further aquifer contamination, but will often prove unsuitable for analysis of existing contamination. For situations where exact knowledge of contaminant pathways is not needed (i.e. land-use planning), the CFR or vertically-averaged 2-D approaches should prove adequate. For cases where vertical variability in hydraulic conductivity is less than an order of magnitude, the 2-D approaches should also be sufficient for prediction of actual contaminant paths. In any other setting, more detailed modeling will be required. Since many hydrologic studies are simply part of continuous aquifer management efforts, the final goal at most sites should be a detailed 3-D understanding, and initial 2-D studies should be designed accordingly.

Finally, understanding the probable effects of facies distributions in alluvial aquifers can give important insights into probable transport behavior in these systems, and quantitative 3-D modeling may not always be necessary. As shown for the Hays case, facies variations largely control transport behavior in the alluvial aquifer. This knowledge can be used independently to estimate high contamination susceptibility zones, or to refine the results of 2-D susceptibility modeling. For example at Hays the common occurrence of lower-permeability silt facies (Qt) overlying the sand facies (Qal) results in semi-confining conditions for the sand. Development of surface contour maps for the sand facies, and determination of those areas where the sand facies extends above the water table could be used to determine zones of greatest aquifer susceptibility to contaminants released above the water table (e.g. Fig. 21). Such areas might warrant protection similar to those with known rapid travel time to water-supply wells, since any contaminant in the unsaturated zone might rapidly infiltrate to the producing aquifer. In effect this would be a WHPA based on a qualitative 3-D capture zone delineation.

Figure 21: Areas with unconfined Qal. Left image shows areas with unconfined Qal (yellow), where water table is found below Qt-Qal contact. Blue color shows confined areas of Qal. Right image shows travel time from water table to drinking water wells. Rapid travel time areas (e.g. red = 5-years) are strongly correlated with down-gradient margins of unconfined Qal.

Many communities only become aware of the need for aquifer characterization after the loss of some groundwater resources by contamination. In these cases, efforts such as WHPA delineation should proceed, but it must be recognized that the delineation results may be inadequate for analysis of specific projected or existing contaminant releases. WHPA delineation as normally practiced, and as affordable for many municipalities, gives an excellent averaged view of contaminant migration potential. It is a very cost-effective way to prevent many future contaminant impacts on drinking groundwater supplies. Analysis of individual contaminant plumes involves additional engineering and legal aspects, and will normally require additional modeling. Detailed 2-D modeling (i.e. beyond that possible with the WHPA computer program) during the delineation process can sometimes bridge this gap, but ultimately, detailed 3-D models will often be needed. In alluvial aquifers, facies-based 3-D conceptual models may supply enough detail, avoiding the need for time-consuming 3-D numerical modeling.