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The effectiveness of planning efforts for groundwater sustainability is directly related to the degree of accuracy reflected in the answers to the following questions.
- Where is the water?
- How much is there?
- Are we using too much?
To address these needs KOT Environmental Consulting Inc. (KOT-ECI) has developed a series of procedures that are copyrighted under the name "STRATMOD"®. The answers to these questions become the basis for developing a groundwater flow model increasing our knowledge of the aquifer system. The results provide the means to conduct cause and affect analyses for various scenarios encountered during the planning process.
Most KOT-ECI projects are completed in a relatively short period of time. This occurs as a result of using data that are readily available in the form of water well and other logs. This information needs to be in the form of an electronic database. If an electronic database is not available, we will prepare one as the project’s initial task. This is necessary for KOT-ECI to readily query the information, add key data, and prepare a stratigraphic model.
Where is the water?
This question is best answered by building a detailed stratigraphic model that can be queried as opposed to a schematic conceptual model loosely based on the basin stratigraphy.
Using STRATMOD ® subjective strata descriptions are changed to objective numerical descriptions facilitating information analysis by computer. Each strata description in the database is replaced with an equivalent value of average hydraulic conductivity. In addition, each stratum is assigned a hydrogeologic unit based on its average hydraulic conductivity. Although populating the database with this information is relatively time consuming, the resulting mapping process is relatively fast.
The stratigraphic model is essentially a stack-map consisting of horizontal layers showing the distribution of the hydrogeologic units at a given elevation. Theses layers are usually spaced at a 5 to 20-foot interval from the surface to lower extent of the available data. Typical models usually contain well over 100 layers.
Once completed, the model can be queried to show the location and extent of the aquifers and the unsaturated aquifer materials as well as aquitards and aquicludes. This information helps to determine areas for future groundwater extraction. In addition, the model is used to identify areas suitable for recharge. Reviewing the model results in an answer to the question, “Where is the water?”
How much is there?
Once saturated aquifer locations are identified, estimates of the total and available volume of water are calculated from hydrogeologic unit areas measured from each layer. These areas are multiplied by the distance between layers and summed to yield the total volume. Applying average total and effective porosity values to this volume and then converting to gallons results in an estimate of total and available water volume or specific capacity and specific yield.
Once completed, this provides the answer to the question, “How much is there?”
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Are we using too much?
A project water budget is developed to answer the question, “Are we using too much?” The following are the elements of a water budget from a recent KOT-ECI project:
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Public records and available literature are used to estimate values for the first four elements. Horizontal inflow and outflow are estimated by conducting a detailed review of available water level and hydraulic property data. Once completed, the stratigraphic model is used to create cross-sections of the study area boundaries. Aquifer units at the boundaries are identified and the cross-sectional areas are measured. The water level data are used to establish ground water flow directions and gradients at the boundaries. This information coupled with the hydraulic property data provides an estimate of groundwater velocity. Velocity information combined with aquifer cross-sectional areas provides an estimate of the recharge rate. If adding the inflow elements and subtracting the outflow elements yields a positive result, adequate water is available for current needs. If negative, there is a water deficit.
Additional maps generated using the STRATMOD ® are used to characterize other attributes of the shallow aquifer system. The types of maps typically produced to estimate additional attributes of the shallow aquifer system include:
- Bedrock Topography
- Depth to Bedrock
- Aquifer Material Topography
- Aquifer Material Thickness
- Distribution of Well Yields
- Potentiometric Map
- Vertical Flow Between Aquifers
- Recharge Map
