Hydrogeological Units OGC GroundWaterML 2 – GW2IE FINAL REPORT

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7. Conceptual Model

The GWML2 conceptual model is designed to be technology-neutral, and focused on the semantics of the groundwater domain. It consists of five components, as well as related properties and other entities: hydrogeological units, fluid bodies, voids, fluid flow, and wells. Conceptually, these entities form a simple template for a subsurface water container: the fluid container a unit or its materials, the fluid itself fluid body, the spaces in the container occupied by the fluid void, the flow of fluid within and between containers and their spaces flow, and the natural and artificial artifacts used to withdraw, inject, or monitor fluid with respect to a container wells, springs, monitoring sites. Well construction details are excluded from the conceptual model, but are included in the logical model for two reasons: 1 thematic, inasmuch as well construction was considered on the periphery of groundwater science, but important to resource management, and 2 practical, as it is sufficiently modeled in GWML1 and could thus be directly imported with few changes. This eliminates the need for its re-conceptualization in the GWML2 conceptual model, thereby keeping it parsimonious.

7.1 Hydrogeological Units

These are distinct volumes of earth material that serve as containers for subsurface fluids. The boundaries of a unit are typically discriminated from those of another unit using properties related to the potential or actual ability to contain or move water. The properties can be geological or hydraulic, and typically include influences from the surrounding hydrological environment. More specifically, the conceptual model delineates two types of hydrogeological units, with slightly different orientations: aquifer-related units have boundaries delimited by the hydrogeological properties of the rock body, while groundwater basins have boundaries delimited by distinct flow regimes. Aquifer-related units are subdivided into aquifer systems, which are collections of aquifers, confining beds, and other aquifer systems. Confining beds are units that impede water flow to surrounding units, and supersede notions such as aquitards, aquicludes, and aquifuges, which are not included herein, as it is difficult to differentiate these in practice. Several significant properties are typically attributed to hydrogeological units, such as porosity, permeability, and conductivity, but these and others are modeled more accurately here as occurring necessarily concurrent with dependent on voids or fluid bodies. For example, porosity, in its various forms, requires both the presence of a unit container and its voids, as it is typically defined as the proportion of void volume to total unit volume i.e. volume of solid material plus voids. Likewise, properties such as hydraulic conductivity and yield require the presence of units and fluid bodies, as they are concerned with the rate of movement of a fluid through a unit. Note that permeability and hydraulic conductivity are differentiated here: permeability refers to intrinsic permeability, which measures the ability of a unit to host fluid flow, independent of fluid properties and based solely on the connectivity and size of voids, whereas hydraulic conductivity additionally considers fluid properties. 27 Copyright © 2016 Open Geospatial Consortium Likewise, management areas are also relational entities in the sense that they are typically necessarily linked with a unit or system and possibly a fluid body. Management areas are earth bodies identified for groundwater management purposes and their boundaries can be delineated by social factors, such as policy or regulation, in addition to physical factors related to hydrogeology or hydrology.

7.2 Fluid Bodies