After water enters an aquifer in a recharge area it flows under saturated conditions to discharge areas controlled by the hydrogeologic characteristics of the aquifer. Gravity drives groundwater flow. The rate (velocity) of groundwater flow is determined by the hydrogeologic parameters (primarily permeability and gradient) of the flow path. The time it takes for water to circulate through an aquifer can range from a few days in a shallow, permeable aquifer to thousands of years in deep aquifers of a groundwater basin. The saturated geologic units within a structural groundwater basin constitute the framework of groundwater flow.
Groundwater flow rates through aquifers and confining units range from very high to very low to essentially no-flow. The flow rate through the pores in a gravel-rich, highly permeable aquifer or through the large open conduits in a carbonate aquifer may be several feet per second (fps), whereas the flow rate within a clay-rich rock unit with very low to essentially no permeability may be less than a few inches every ten thousand years. Thus, flow rates through the various types of hydrogeologic units vary over 11 to 12 orders of magnitude. Folding, fracturing, and faulting modify the permeability and other hydraulic properties of both aquifers and confining units, and generally decrease the capacity of confining units to function as barriers to groundwater flow.
Groundwater occurs under unconfined (water table) conditions in unconsolidated deposits and bedrock formation outcrop areas throughout the WBRB. Shallow, unconfined groundwater recharge, flow, and discharge in the WBRB are predominantly controlled by topography and stream drainage patterns. Local and regional shallow groundwater flow (less than 300-500 feet below the land surface) follows topography, is recharged by precipitation, and is discharged to streams and river drainages. Shallow groundwater flow in areas with hills, uplands, and high topographic relief is generally controlled by these local topographic features. Complex interactions can occur among bedrock aquifers, unconsolidated aquifers, and surface waters, especially along drainages lined with alluvial deposits. The discharge of groundwater to surface drainages contributes to base flow and in some cases constitutes all of base flow.
Groundwater and surface water are interconnected in the WBRB, as in all Wyoming basins. Discharge of groundwater to the surface may occur as described above from springs, subcrop discharge below and adjacent to surface waters, and wells. Surface water bodies recharge groundwater as permitted by the hydraulic gradient and characteristics of the underlying and adjacent earth materials.
Recharge of the deeper Paleozoic and Mesozoic aquifers in the WBRB occurs primarily where they have been up-folded, eroded, and now crop out in the higher-elevation areas around the perimeters of the structural basins. As groundwater flows downdip from the recharge areas into the structural basins, it becomes confined by low-permeability strata (shale, claystone) overlying and underlying the more permeable aquifers (sandstone, coal, fractured limestone and dolomite). Some recharge to deeper aquifers occurs as leakage from adjacent, usually underlying, hydrogeologic units. Some discharge occurs where recharge is rejected from fully saturated aquifers into headwater streams usually at the point where a stream intersects the contact between a confining unit and underlying aquifer. Artesian groundwater flows at the land surface from wells completed in confined aquifers whose hydraulic pressure is greater than atmospheric pressure at the land surface.
Confined groundwater flow within the deeper bedrock formations of the structural basins and adjacent structural uplifts of the WBRB is commonly structurally and stratigraphically controlled. Major aquifers and aquifer systems in the WBRB occur predominantly within interstratified sequences of highand low-permeability sedimentary beds. The WBRB aquifers are commonly heterogeneous and anisotropic on both local and regional scales. Deeper groundwater flow in the WBRB is predominantly through permeable formations down-gradient (from higher to lower hydraulic pressure) and generally downdip toward the axes of the structural basins, and eventually northward into Montana.
Although groundwater flow is driven by gravity, the water does not always flow down hill, but from areas of higher pressure (greater hydraulic head) to areas of lower pressure (lower hydraulic head). In the deep subsurface, flow along any permeable vertical pathway is commonly from lower to higher elevation, as demonstrated by artesian wells and some springs that discharge groundwater from deep aquifers at the land surface. Groundwater flows in the directions indicated on potentiometric surface maps if permeable pathways exist; however, flow through preferential pathways (e.g., fractures) can depart from the maximum gradient direction. Hydraulic gradients are commonly steep in low-permeability geologic units where there is substantial resistance to flow (friction), and shallow (low-angle) to nearly horizontal within highpermeability units where resistance is low – analogous to a standing body of water, such as a pond, where there is no resistance to flow in any direction and the gradient due to gravity is flat.
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