Hydrostratigraphy Aquifer Mapping

Mapping Regional Flow Systems Hydrostratigraphy Hydrogeological Cross Sections Potentiometric Surfaces Water Table Maps Recharge & Discharge Areas Surface Water Interactions

Definition Hydrostratigraphy is the identification of mappable-units on the basis of hydraulic properties (aquifer / aquitard) that have considerable lateral extent and that also form a geologic framework for a reasonably distinct hydrogeologic system. MJR

Hydrostratigraphy Stratigraphic Lithologic Hydrostratigraphic Clay Surficial Deposits Surficial Aquitard Floral Aquifer Floral Fm Floral Aquitard Empress Gp Empress Aquifer Bearpaw Fm Bedrock Aquitard Sand & Gravel Sand Shale / Mudstone Till Clay

Regional Hydrogeology The starting point for any regional hydrogeological characterization study is to establish the hydrostratigraphy by identifying mappable flow units and intervening aquitards. This is done using standard subsurface and surface mapping techniques based of the principles of sequence or genetic-unit stratigraphy. Emphasis is placed on the characterization of hydraulic properties of lithofacies within each genetic-unit on various scales using data from a wide range of sources including thin-sections, core studies, slug tests, DSTs and pump tests.

Aquifers and HSUs Hydrogeologists have long noted that ground water flow often does not conform to the boundaries of recognized stratigraphic units. Two terms, “aquifer” and “hydrostratigraphic unit,” are commonly employed to subdivide the subsurface into units more relevant to groundwater hydrology. The term “aquifer” is commonly defined for water supply usage in economic terms. In many areas, “aquifer” is defined by local laws and regulations which makes it difficult to use as a technical term. The term “hydrostratigraphic unit” (HSU) has been defined in a variety of ways in the literature, and does not currently have a formal definition

Maxey The term hydrostratigraphic unit was first proposed by Maxey (1964) for “bodies of rock with considerable lateral extent that compose a geologic framework for a reasonably distinct hydrologic system.” Maxey (1964) identified the need to define ground water units that are based not solely on specific lithologic characteristics but also included parameters “that apply especially to water movement, occurrence, and storage.”

Seaber Seaber (1982; 1986; 1988) proposed a definition of hydrostratigraphic unit as “a body of rock distinguished by its porosity and permeability,” which he considered more consistent with established stratigraphic nomenclature. With this definition, Seaber intended to accommodate the observation that a “hydrostratigraphic unit may occur in one or more lithostratigraphic units.” Seaber (1988) attempted to define a hydrostratigraphic unit that applied to all geological environments by focusing on the material properties of the rock or sediment.

Lawrence Livermore National Lab LLNL define a hydrostratigraphic unit (HSU) as a body of sediment and/or rock characterized by ground water flow that can be demonstrated to be distinct under both unstressed (natural) and stressed (pumping) conditions. Flow in an HSU is distinguishable from flow in all other HSUs. Using this definition, an HSU is not restricted to any particular geologic setting, which was the intent of Seaber, but is principally based on the properties of ground water flow, which is consistent with Maxey.

Aquifers, aquitards and HSUs An aquifer can be composed of one or more HSUs. Thick aquitards or aquicludes may be defined as HSUs based on their distinct groundwater flow characteristics. Thin aquitards or aquicludes that form significant, laterally continuous layers that limit hydraulic communication may be used to define HSU boundaries.

Hydrostratigraphic Maps Hydrostratigraphic maps and sections superficially look very similar to simple lithostratigraphic maps with two important differences: Amalgamated HSUs have properties controlled by the fabric of their components (eg high K lenses and fractures rather than a simple bulk average). Hydraulic anisotropies in otherwise similar materials may be classified as different HSUs (fracturing or strong transverse anisotropy may define a distinct unit).

Hydrostratigraphic Units Unit #1: Isotropic Aquitard Unit #2a: Anisotropic Aquitard Kv>>Kh Unit #2b: Isotropic Aquitard Unit #3: Isotropic Aquifer Unit #4: Isotropic Aquitard Unit #5a: Fractured Bedrock Aquifer Unit #5b: Unfractured Bedrock Aquifer

Hydrostratigraphic Unit Analysis Hydrostratigraphic Unit (HSU) analysis was developed and implemented at Lawrence Livermore National Laboratory (LLNL). HSU analysis integrates chemical, hydraulic, geologic,and geophysical data to produce a comprehensive three-dimensional model of the subsurface based on fluid flow characteristics. It is the focus on fluid flow pathways that renders this management tool so effective for characterization and remediation activities. HSU analysis significantly enhances the basis of computer models of fluid flow and transport at contaminated sites.

HSU Analysis and Decisions HSU assists decision making : locating and designing characterization boreholes and monitor wells, (2) prioritizing the construction and phased startup of remediation systems, (3) managing extraction of subsurface contaminants, (4) finding sources of past contaminant releases, (5) tracking the migration of subsurface contaminants, (6) evaluating the subsurface effectiveness of the remediation systems.

Hydraulic Heads Hydraulic heads vary in three spatial directions and time, h(x,y,z,t). The time element can be removed if all measurements are made at the same time, h(x,y,z). A further dimension can be eliminated if piezometers are placed where the flow is largely two-dimensional, h(x,y) However, the resulting maps must be viewed as 2D projections of a 3D field.

Potentiometric Surface Maps Hydraulic heads from water wells and piezometers, from a single HSU, can be contoured on map. Such a map for a confined aquifer is called a potentiometric surface map. If the aquifer is confined, the surface is a water table map. These maps assume that flow in the HSU is horizontal and that there is no change in head with depth. Potentiometric surface maps are only valid for HSUs corresponding to aquifers.

Constructing Potentiometric Maps In order to create a potentiometric surface map numerous observations of the elevation of the surface must be made with wells and piezometers. Several assumptions must be satisfied: 1. The wells and piezometers have screens installed that are near the same elevation. A large variation in elevations may lead to problems since both horizontal and vertical gradients exist in aquifers. 2. There must have enough data points to draw a contour map. If only a few points are available it may only be possible to determine the general direction of flow and the hydraulic gradient. 3. Surface bodies of water such as streams, lakes, and springs provide information about the water table in unconfined aquifers.

Dakota Sandstone Black Hills Darton (1909) was responsible for one of the first potentiometric maps for the Cretaceous Dakota Sandstone in South Dakota.

Flow Nets K1 K2 Dh A1 L1 A2 L2 Q Equipotentials and flow lines form a network called flow net. For a flow net flow all tubes carry the same flow and there is no flow normal to flow lines. This means: Q = K1DhA1/L1 = K2DhA2/L2

Interpreting Flow Nets For a uniformly thick aquifer A1 = A2 and Dh is constant for contours at equal intervals. It follows that: K1 = L1 K2 L2 Thus if K1 has been measured K2 can be readily estimated for an isotropic but inhomogeneous aquifer.

Interpreting Potentiometric Surfaces Increasing gradients mean: Increasing flux (in recharge area) Reducing Kh (aquifer becoming less conductive) Reducing thickness (aquifer thinning)

Interpreting Potentiometric Surfaces Recharge and Discharge Increasing flux (in recharge areas) Decreasing flux (in discharge areas) Flow has a downward component in recharge areas Flow has an upward component in discharge areas

Discharge to Surface Water System Streams Mainly discharge features (some streams can lose flow ) Springs where the water table intersects the ground surface associated with conductivity changes Seepages Salinity increases for seepages and springs in arid climates due to evaporation