Ground Water Level Monitoring -- Part 3
Water level data are collected over various lengths of time, dependent on their intended use(s). Short-term water level data are collected over periods of days, weeks or months during many types of ground water investigations. For example, tests done to determine the hydraulic properties of wells or aquifers typically involve the collection of short-term data. Water level measurements needed to map the altitude of the water table or potentiometric surface of an aquifer generally are collected within the shortest possible period of time so that hydraulic heads in the aquifer are measured under the same hydrologic conditions. Usually, water level data intended for this use are collected over a period of days or weeks, depending on the logistics of making measurements at different observation well locations.
Long-term water level data are fundamental to the resolution of many of the most complex problems dealing with ground water availability and sustainability. As stated previously, significant periods of time -- years to decades -- typically are required to collect water level data needed to assess the effects of climate variability, to monitor the effects of regional aquifer development or to obtain data sufficient for analysis of water level trends.
Many of the applications of long-term water level data involve the use of analytical and numerical (computer) ground water models. Water level measurements serve as primary data required for calibration and testing of ground water models, and it often is not until development of these models that the limitations of existing water level data are recognized fully. Furthermore, enhanced understanding of the ground water flow system and data limitations identified by calibrating ground water models provide insights into the most critical needs for collection of future water level data. Unfortunately, this second step of using ground water models to help improve future water level monitoring rarely is taken.
Data-collection ProgramsTo aid in preparation of this report, state and local water resources agencies and United States Geological Survey (USGS) district offices were asked to provide information about the design, operation and history of long-term ground water observation wells in their respective states. "Long term," as defined here, refers to any well being used to collect water level measurements for five years or more, or having at least five years of hydrologic record. It is worth repeating that water level measurements typically must be collected from an observation well without interruption over one or more decades in order to compile a hydrologic record that represents the potential range of natural water level fluctuations and tracks trends over time. Five years is, therefore, a relatively short period for water level data collection, but it is at least sufficient to provide a record of several seasons of ground water level fluctuations.
Sixty-two state and local water management or regulatory agencies provided information, as did USGS offices in all 50 states and Puerto Rico. A surprising revelation was how difficult it is to obtain information about the actual number of observation wells monitored, the frequency of water level measurements, the average period of hydrologic record and changes in the monitoring program over time. The reasons for this varied, but often the ability of the respondents to provide information was hindered by a lack of formal documentation about the design of the observation well networks, limited "institutional memory" and the lack of an accessible database. Another common problem encountered was that responsibilities for collecting water level data are not always clearly defined.
The level of effort in collecting long-term water-level data varies greatly throughout the United States. Although difficult to define precisely, the information collected indicated that there are on the order of 42,000 long-term (five or more years of record) observation wells distributed throughout the United States. Approximately 11,000 (slightly more than one-quarter) of the reported number of long-term observation wells presently are monitored through the USGS Cooperative Water Program.
In many states, a lack of sufficient financial resources impedes the construction of new observation wells in areas of need. To eliminate costs incurred by drilling and well construction, most agencies use private water wells or existing monitoring wells for the collection of water level data. These "wells of opportunity" often are useful as long-term observation wells, but a problem reported by many states is the difficulty in locating suitable existing wells in specific aquifers or geographic locations. Limitations in funding and staffing also impair observation well maintenance, upgrades to water level monitoring equipment and consistency in water level monitoring activities conducted from year to year.
A proper evaluation of the suitability of existing observation well networks is best done at the state and regional level, where the diversity in topographic, climatic and geologic settings, ground water use and other factors can be properly considered. Two indicators of the status of observation well networks are presented here that may be useful in comparing the approximate magnitude of long-term observation well networks by state or region. The first indicator, observation well density, is the ratio of the reported number of long-term observation wells in each state to the area (in 1,000 square miles) enclosed within state boundaries. The second indicator, which relates water level data collection to ground water use, is the ratio of the reported number of long-term observation wells to the total amount of ground water withdrawn (in 100 million gallons per day) from each state.
The information presented on the map above provides some indication of the relative magnitude of long-term ground water level data collection in various parts of the nation. The data do not indicate the degree to which observation wells are distributed geographically and among aquifers in any particular state. Large observation well networks in states having comparatively high values of one or both indicators may be good candidates for network evaluation designed to determine if monitoring sites may be reduced or redistributed to enhance data collection or reduce operational costs. Conversely, comparatively low values of one or both indicators generally reflect a sparse number of wells relative to geographic area or to ground water use in the indicated state. In these cases, in particular, a larger number of observation wells may be needed to ensure that sufficient water level data are being collected, at a minimum, where ground water withdrawals are concentrated or where sensitive environmental areas are located.
As with stream flow and precipitation data, ground water level data become increasingly valuable with length and continuity of the records. Yet, unlike stream flow and meteorological records, ground water level records in most parts of the nation are less than 40 years in length. Forty-four percent of agencies reported having observation well networks in which the typical hydrologic record was 25-40 years, 31 percent reported having observation well networks in which the typical hydrologic record was 10-25 years, and 2 percent reported having networks in which the typical hydrologic record was less than 10 years. Twenty-two percent of the agencies reported that observation wells in their networks had periods of hydrologic record too varied to characterize.
In recent years, the USGS and many state and local agencies have experienced difficulties in maintaining long-term water level monitoring programs because of limitations in funding and human resources. Where fiscal or personnel constraints have forced agencies to revise priorities for environmental data collection, preference typically has been given to water quality monitoring, often at the expense of basic ground water level monitoring. Although water level and ground water quality monitoring are complementary activities, these two types of data commonly are treated as mutually exclusive, and separate agencies commonly are responsible for each. Greater attention is needed to the long-term value of water level data collected as part of water quality monitoring and to the potential synergies between water quality and water level monitoring networks.
In many states, observation wells tend to be concentrated in areas where aquifers are developed heavily. Few long-term observation wells are intentionally located away from the influence of pumping, irrigation and other human activities to allow for monitoring of the natural effects of climate variability and to provide baseline data against which ground water levels monitored during short-term investigations can be better evaluated in a longer term climatic perspective. The USGS presently operates a sparse national network of about 140 climate-response wells, and a few states have drought monitoring networks that include climate-response observation wells. Increased numbers of climate-response observation wells and long-term monitoring of naturally occurring fluctuations in ground water levels are needed to develop more complete ongoing assessments of droughts and the cumulative effects of other climatic phenomena. During drought conditions, the effective management of ground water resources, and monitoring of ground water availability and ground water and surface water interaction, require the ability to rapidly collect water level measurements and track trends. Therefore, more efforts should be made to construct climate-response and other observation wells capable of collecting "real-time" water level measurements, and to make all collected water level data more rapidly and readily accessible through electronic transmittal.
Challenges and Future OpportunitiesThe focus of this report has been to illustrate the importance of systematic, long-term collection of water level data. Such data are crucial to the investigation and resolution of many complex water-resources issues commonly faced by the industry and the public. To ensure that adequate water level data are being collected for present and anticipated future uses, observation well networks and water level monitoring programs at the local, state and federal levels need to be evaluated periodically. In the course of these evaluations, several questions might be asked. Are data being collected from areas that represent the full range in variation in topographic, hydrogeologic, climatic and land-use environments? Are plans to ensure long-term viability of observation well networks and data collection programs being made? How are the data stored, accessed and disseminated? Who are the principal users of water level data, and are the needs of these users being met?
Careful planning and design are required to ensure the collection of high-quality water level data over the period of time needed to compile a useful hydrologic record of water level changes. A further challenge is to supplement the long-term monitoring wells as hydrologic conditions in aquifers evolve. A comprehensive monitoring program should consider aquifers substantially affected by ground water pumping, areas of future ground water development, surficial aquifers that serve as major areas of ground water recharge, and links with water quality and surface water monitoring.
A commitment to long-term monitoring is needed to avoid data gaps resulting from an inadequate distribution of observation wells or periods of no measurements in a hydrologic record. Many agencies lack formalized written plans for the design and operation of ground water level networks, and many agencies have difficulty maintaining the funding and program continuity necessary to ensure long-term collection of water level data. This can hinder the ability of water resources managers to make sound resource-management decisions. Where water level data are not available, hydrologic information needed to address critical ground water problems may be impossible to obtain. Much recent effort has been made in the application of computer modeling techniques to forecast future ground water levels. However, the successful application of even these advanced methods requires that sufficient water level data are available.
More effort is needed to increase the amount of ground water level data stored in electronic databases, to increase the compatibility between databases and to improve access to ground water level data on the Internet. Although some water level databases can be accessed in this way, detailed and complete records of historical water level data usually are limited or unavailable. In many agencies, large backlogs of historical ground water level data have not been entered into electronic databases, let alone made available on the Internet. Consequently, potentially useful data are residing in paper files where accessibility and utility are very limited.
Finally, to increase the collection and accessibility of water level data, agencies need to examine ways to increase interagency coordination in constructing and maintaining observation well networks, collecting water level measurements, and sharing and disseminating data. Greater interagency cooperation will help ensure that data collection efforts are sufficient to address issues relevant to the greatest variety of local, state, regional and national water resources issues.