< PreviousMGMT User’s Manual Version 1/28/2019 10:38 PM the feature layer. The distribution of Type I or Type II water supply wells may be viewed by expanding the WELLS layer group and activating the appropriate well layers (see Figure 46). Management of groundwater is often aided by considering different components of the hydrologic cycle (precipitation, baseflow, etc.) across an entire drainage basin – or watershed. Both watershed and subwatershed layers are available as Default Layers in MGMT. As shown in Figure , it is possible that the study area encompasses multiple watersheds and that the different watersheds are managed separately by different municipalities or local agencies. On the other hand, a single watershed may cut across different political jurisdictions, each with its own management oversight. Therefore, understanding the different watersheds that drain the study area and involved stakeholders is a key first step in any regional groundwater study. Another key aspect of the landscape is the topography and surface water network, and in particular, the distribution of regional highlands and lowlands. The highlands tend to be where regional groundwater systems are recharged and where headwaters of streams are located, whereas lowlands are typically dissected by water courses (discharge areas) and may be prone to flooding in extreme precipitation events. Characterizing the topography and surface water network at any scale is therefore a quick way to better understand the groundwater system at that scale. An example for Ottawa County is shown in Figure 47. Figure 45: Using the County and Township layers to visualize population across the state and Ottawa County. + Population by Township (2000 census) Population by County (2000 census) Identify areas of high population will have a higher water demand Consider population and implications for water use relative to other areas of the State of Michigan. MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 46: Locations of Type I and Type II water supply wells, and political boundaries of cities and villages in Ottawa County. Figure 47: Mapping major watershed boundaries at the regional and local scales. Visualize the distribution of Type I and Type II water supply wells Display political boundaries of cities and villages Understand the different watersheds that drain the study area and consider how regional management may different across jurisdictions or watershed Ottawa County consists of four major watersheds MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 47: Visualizing the land surface topography and surface water network at multiple scales. Elevation (m) Elevation (m) Elevation (m) Identify regional and local topographic features and the surface water network and flood-prone areas Local highlands Land surface towards the eastern and southern end of the County is fairly rugged, undulating, and dissected by water courses. Low, flat plains in the lower reaches of the Grand River and much of the central part of the County MGMT User’s Manual Version 1/28/2019 10:38 PM 3.2 Characterizing the Geology Geology provides a qualitative knowledge of the framework of flow - the three-dimensional configuration of geologic deposits through which flow takes places is a primary control of the regional flow dynamics (Freeze, 1979). Groundwater investigators in Michigan may need to consider both the unconsolidated glacial sediments in the near-surface environment as well as the deeper bedrock structure consisting of alternating layers of aquifers and aquitards. Ottawa County is located on the WSW margin of the Michigan structural basin, so the bedrock formations underlying the county strike NW – SE (see Figure ). Only three sedimentary formations subcrop beneath Ottawa County: the Coldwater Shale (confining material), the Marshall Formation (aquifer) and the Michigan Formation (partially confining material). The Marshall Formation serves as a conduit for deep regional groundwater flow to slowly upwell toward the surface. The distribution of glacial land systems in Michigan controls the shallower regional groundwater flow, including relative infiltration of precipitation and/or surface water to the aquifer system. The two-dimensional distribution of major land systems in Ottawa County is shown in Figure 48. Areas dominated by coarse-textured sands and gravels (e.g., the sand dunes and outwash in Fig. Figure 48) are more pervious than the areas dominated by fine-textured glacial till and lacustrine deposits. This suggests significant variability in recharge to the aquifer system across Ottawa County (discussed more below). Of course, there may be significant three-dimensional variability associated with the unconsolidated glacial sediments, and so it is useful to supplement the plan-view mapping of major glacial land systems with vertical cross-sections. Examples for Ottawa County are shown in Figure 50. Well lithology information is used to differentiate aquifer material classes in the drift and bedrock wells available in the WELLS layer group. The vertical cross-sections reveal that central Ottawa County has a rather continuous confining layer overlying the bedrock aquifer, likely restricting freshwater recharge from the surface to the bedrock in this area. On the other hand, wells along the shoreline benefit from a thick surficial layer of aquifer material overlying the Coldwater shale (confining material). The boreholes can also be displayed in fully three-dimensional form using the Create 3D Model tool described in section 2.6.2 Integrated 3D Overlays. This helps to holistically evaluate intra-aquifer heterogeneity across the study domain, and when viewing the layer from below, it also allows for examination of bedrock wells with respect to the bedrock top elevation surface (see Figure 50). MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 49: Mapping the deep bedrock geology in the horizontal and vertical directions. Marshall formation (aquifer) Coldwater Shale (aquitard) Michigan Formation (aquitard) Visualize the regional and local bedrock geology in the plan and cross-section views The County is underlain by three bedrock nits, including a deep, fractured bedrock aquifer General movement of deep regional groundwater MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 48: Visualizing the major glacial land systems in the study area. Glacial land systems Coldwater Shale Marshall Sandstone Michigan Formation Glacial drift Glacial land systems Identify the distribution major glacial land systems, which has implications for recharge to the water table and leakance to the bedrock aquifers (e.g., fine-grain material impedes surface recharge) Lacustrine Sand dunes Till moraines Outwash Lacustrine clays MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 49: Mapping the distribution of drift and bedrock wells and using cross-sections to examine lithology structure in wells. Glacial drift layer Bedrock (mostly Coldwater Shale – CM) Bedrock (mostly Marshall Aquifer –AQ) Glacial drift layer AQ MAQ PCM CM Examine the distribution of bedrock wells (blue) and glacial wells Use cross-sections with the well layer visible to examine lithology structure in the subsurface Along the western potion of the County, many glacial wells and bedrock wells are absent Wells in central portion of the County must penetrate through the extensive confining layer to draw water from the aquifer MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 50: Three-dimensional visualizations of the intra-aquifer variability of geologic materials in the glacial drift aquifer. AQ (aquifer) MAQ (marginal aquifer) PCM (partially confining) CM (confining material) AQ (aquifer) MAQ (marginal aquifer) PCM (partially confining) CM (confining material) Visualize the 3D distribution of material types in the glacial drift aquifer Visualize well lithologies from different angles, including from below Bedrock wells are nearly absent from the western portion of the County where the Coldwater shale subcrops Heterogeneous mixture of aquifer and confining material along the eastern portion of the County. Extensive confining material in the central portions of the County MGMT User’s Manual Version 1/28/2019 10:38 PM 3.3 Understanding the Groundwater System Ultimately, almost all groundwater studies will require a delineation of the groundwater elevations in the surficial and/or bedrock aquifers. Groundwater flow through saturated porous media is governed by Darcy’s Law, which states that the specific discharge, q, [L/T] is proportional to the hydraulic gradient, i: 푞=−퐾푖 where the constant of proportionally, K, is the hydraulic conductivity of the aquifer. (Note that the negative sign is to ensure positive flow is in direction of the hydraulic gradient). For users of MGMT, a spatially-explicit hydraulic conductivity raster is available (see section 2.5.1 and 2.5.2). It is useful to map the hydraulic conductivity across the study domain to characterize the relative quality of yield and rate of groundwater movement, as specific discharge is proportional to K (see Figure 51). The hydraulic gradient is estimated using the methods described in section 2.5.1. In any groundwater study, it is important to have consider both regional and subregional flow patterns. Certain patterns may not be discernable when viewed at larger scales, although their recognition at smaller scales may spark insights and new hypotheses. Understanding detailed local dynamics, however, often requires an understanding of the regional context, i.e., taking into account the regional controls on the groundwater system. This is illustrated in Figure 52, in which a regional model is developed to identify regional discharge and recharge areas, and a more detailed local model is developed in the study area to identify local discharge and recharge areas. Figure 51: Mapping hydraulic conductivity across the study domain. Hydraulic Conductivity (ft/d) Map the hydraulic conductivity to gauge the quality of yield and movement of water in the glacial drift aquifer Low yields Higher yields Higher yields Higher yields Higher yields Low yields MGMT User’s Manual Version 1/28/2019 10:38 PM Figure 52: Delineating regional and local SWL distributions to identify recharge areas and discharge areas. Next >