|skip nav rules forms FAQs reports board meetings OCWP|
What is an aquifer? What is groundwater?
An aquifer is a rock layer or group of rocks (formation) that has the ability to hold and move water underground. This water is held in pores between grains such as sand in a sandstone or it is held in fractures if no pores are present like a limestone. Not every rock or formation is an aquifer, but all have the capacity, if the conditions are right, though some are better than others. In Oklahoma the sandstone aquifers that dominate the Western part of the state tend to be more reliable and productive than the limestone aquifers that dominate the Eastern portion of our state. This is the reason that Eastern Oklahoma relies much more on surface water (rivers, lakes, etc.).
The OWRB separates aquifers into two different categories, major aquifers and minor groundwater basins; within those categories are bedrock and alluvial aquifers. The major aquifers produce greater amounts of water at least 50 gallons per minute for bedrock aquifers and 150 gallons per minute for alluvial aquifers. The minor basins produce significantly less amounts of water. The terms alluvial and bedrock are mainly descriptive of the proximity the aquifer is to a river. The alluvial aquifers are products of river movements over thousands of years. These loosely cemented materials recharge from the river, the degree by which can be directly influenced by the water levels in the river. Bedrock aquifers are typically older, more cemented, and larger than the alluvial aquifers. Their recharge is often slower and vary over time.
For more information on groundwater and aquifers visit the USGS Water Science School website.
The first step in any hydrologic investigation is characterization of the resource. Initially, upper, lower, and lateral boundaries of the groundwater basin are determined, and then aquifer properties—such as saturated thickness, hydraulic conductivity (ft/day), transmissivity (ft²/day), specific yield, and storage coefficient—are determined to understand the storage and yield capacity of the basin. Groundwater basins are dynamic; aquifers adjust constantly to short-term and long-term changes in climate, groundwater withdrawals, and land uses. Thus, the amount of water entering the basin (recharge) and the amount of water leaving the basin (discharge) are also important factors in the evaluation of groundwater availability.
Hydrogeologists use a variety of methods to estimate aquifer parameters, recharge, discharge, and water quality of the basins. These include physical measurements, such as water levels in wells, stream flow measurements, aquifer pumping tests, tracer tests, and water quality sampling. Other methods are indirect such as developing water budgets and utilizing computer groundwater flow models. Computer models enhance our understanding of current conditions as well as assist in the prediction of aquifer response to future climatic or land use changes.
Consistent with state law, the OWRB conducts maximum annual yield (MAY) studies to determine amounts of water that may be withdrawn from Oklahoma's groundwater basins by permitted water users. The resulting figure is considered to be the amount of water that can be safely withdrawn from an aquifer to ensure a minimum basin life of 20 years.
To arrive at a basin's MAY, investigators map the total land overlying the basin-often divided into sub-basins for yield determinations-and estimate the amount of water in storage. Next, they determine the rate of natural recharge and total discharge, transmissivity, and potential for pollution from natural sources. The balance of the available water is then allocated proportionately to each acre of land overlying the basin. Prior to final consideration of this prorated amount, hearings are held to allow public input to be included into the determinations.
©1998-2013, Oklahoma Water Resources Board
Page last updated: February 28, 2013