< PreviousMGMT User’s Manual Version 1/28/2019 10:38 PM The chemical and biochemical quality of groundwater determines its usefulness for domestic purposes and drinking water supply, agriculture, and industry. Identifying trends in water quality (both in space and time) is, therefore, of importance to most groundwater studies. In Ottawa County, reports of groundwater salinity elevated above natural conditions prompted an investigation into chloride (Cl-) concentrations (as a proxy for salinity). Figure presents a plan-view and 3D views of Cl- concentrations in Ottawa County generated using the approach described in section 2.2.3. It may also be helpful to visualize concentrations above select threshold values to better understand spatial trends and locate samples showing very high concentrations (see Figure ). It is clear that the highest concentrations occur in the central part of the County and along stream corridors, whereas concentrations are generally low along the coastline and in the NE and SE regions. Along stream corridors groundwater is generally moving upward to discharge into the surface water bodies, suggesting a relationship between the groundwater hydrology and salinity. Indeed, when Cl- concentrations are overlaid to the groundwater elevation surface, high concentrations are focused to areas of lowest SWL – discharge zones (see Figure 54). Users of MGMT can filter the chemistry concentrations data by checking the ‘Time phrase within’ box when generating Chemistry concentration data (see Figure ). A series of graphics – each filtered for a different time period – can reveal temporal trends embedded in the spatial chemistry concentration datasets. An example is provided in Figure 59, which shows how the distribution of elevated Cl- concentrations have changed across decades in Ottawa County. MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 53: Visualization of spatial variations in water quality (chloride concentrations) in the study area. Cl- concentration (mg/L) Map groundwater quality spatial dynamics across the study domain This figure shows the chloride concentrations in water samples from wells tested 1980 through 2010. Concentrations are elevated well above background levels in the state, especially in the central part of the County and in and along stream corridors. Visualize water quality in 3D, using an offset to locate the samples in the subsurface MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 56: Visualizing concentrations above select threshold values. Figure 57: Probing the relationship(s) between water quality and hydrogeology. Visualize concentrations above select threshold values to better understand spatial trends and locate samples showing very high concentrations. The red samples are locations where concentrations exceeded the threshold; blue samples are locations where concentrations were below the threshold. Overlay water quality on the potentiometric surface to identify relationships between water quality and hydrogeology. Note that virtually all drinking water exceedances in the County occur in the groundwater discharge areas, where SWLs are low and groundwater moves upward. Cl- concentration (mg/L) MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 58: Filtering chemistry concentration data by a time period or by value range. Filter chemistry concentration data by a time period. Filter chemistry concentration data by a value range Assign a ‘No Data’ value (default = 0) MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 59: Visualizing elevated chloride concentrations in the study area for distinct time periods (1980-1999; 2000-2004; 2005-2010). 3.4 Protecting Drinking Water Supply Safe-guarding public groundwater supply systems from contamination is an important goal for anyone interested in community development, environmental protection and drinking water quality. Determination and management of wellhead protection areas can be done in a streamlined fashion by using the MGMT modeling environment, allowing the user to naturally progress from WHPA delineation to identifying contamination sources and assessing the aquifer’s ability to naturally disperse pollutants. This process is illustrated by considering an existing Type II drift well in northeast Ottawa County (see Figure 60). The first step is to delineate the local flow patterns around the site using the appropriate SWL dataset. Drift wells and surface water features were using to generate the 2D model of the potentiometric surface in the glacial drift aquifer, which is shown in Figure 61. Also shown are WHPAs delineated for 3-, 5-, 10- and 20-year time of travel. Although the 10 years is the time period selected for the Michigan Wellhead Protection Program, it may be useful to use other time periods depending on the locations of potential/known contamination sources and other environmentally-sensitive receptors (e.g., wetlands). The nearby potential sources of contamination for this particular example are shown in Figure 62. Included in MGMT are layers featuring: underground storage tanks (UST), including leaking USTs; waste handling facilities; historical and regulated landfills; oil and gas wells; and known contamination sites from manufacturing and industrial facilities, commercial/institutional facilities, or utility facilities. Users may also add their own shapefiles of other potential sources of contamination, e.g., septic Visualize spatio-temporal trends of concentrations in concentrations above select threshold values. The red samples are locations where concentrations are above 250 mg/L; blue samples are locations where concentrations are below 250 mg/L. This graphic suggests the chloride contamination is becoming worse with time. MGMT User’s Manual Version 1/28/2019 10:38 PM fields, abandoned wells, or agricultural operations. In addition, it is useful to the map of chemistry concentration of contaminants derived from human activity (e.g., nitrate from fertilizer application – see Figure 62) or that occur naturally in the subsurface (e.g., arsenic). The locations of surface water bodies near the supply well is shown in Figure 63. The relative ability of the aquifer to disperse a contaminant plume (and thereby reduce concentrations in the groundwater) can aid in the prioritization of resources dedicated to groundwater supply protection. The rate of freshwater infiltration and the overall volume of the aquifer controls the dispersion (higher infiltration and larger volume leads to greater dispersion). Infiltration is controlled by the near-surface geology and volume is controlled by aquifer thickness. Using the cross-section tool in MGMT can help users determine the near surface aquifer material types in the vicinity of WHPA as well as the aquifer thickness. In this example, the near- surface is dominated by coarse lacustrine deposits of relatively high hydraulic conductivity (Figure 64). Moreover, the glacial layer is underlain by fractured sandstone, resulting in a relatively thick aquifer system (Figure 65). Together, the strong connection to the surface and multi-layer aquifer configuration results in relatively resilient local aquifer conditions with respect to contamination. MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 60: (top) Type I and Type II drift and rock wells located in Ottawa County, MI; (bottom): Type II drift well selected for an illustrative example of WHPA determination and management. MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 61: (top) flow patterns underling the site; (bottom) WHPA delineated for 3 years, 5 years, 10 years, and 20 years of groundwater travel time. 20 year WHPA 10 year WHPA 5 year WHPA 3 year WHPA SWL (m) 190.9) 176.8 MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 62: (top) locations of potential sources of groundwater contamination; (bottom) nitrate concentrations in water wells. Nitrate concentration (mg/L) Potential sources of Contamination MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 63: Surface water bodies near the WHPAs. Wetlands Lakes Streams Next >