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Historical Guidelines for Index Well Selection Within a CBR Funded Network

Historical overview of selection of initial CBR-funded wells

(This page provides information for historical reference only. Refer to current guidelines for up-to-date guidelines and contact information.)

Background

The responsibility for allocation of federal Collection of Basic Records (CBR) funds for groundwater-level monitoring has been transferred to the Office of Groundwater (OGW) which has assembled a Groundwater-Level Monitoring Task Group to make recommendations for ways to improve USGS groundwater-level monitoring networks. The Task Group met in September and December 1995 and recommended that the CBR funds be directed to monitoring water levels in wells that respond to natural climatic changes and that the current network be reevaluated for this purpose.

The Ground-Water-Level Monitoring Task Group recognized that the CBR funded network is a small part of a broader nationwide water-level monitoring network. Because of limited CBR funding levels, the Task Group considered this effort a first step in a long-term process of network evaluation and design in which the Office of Groundwater should provide leadership.

Local interest and funding for water-level monitoring networks varies widely across the country and typically is driven by concerns for human-induced changes. A stable, long- term network that reflects climatic changes fills a unique national need that the Survey has not been successful in building and maintaining with cooperatively funded networks. This network is useful for (1) placing data collected during short-term projects in long-term climatic perspective, (2) providing a current index to drought conditions, (3) making long- term estimates of natural recharge and its temporal variability, and (4) filtering out climatic effects from water-level records affected by human-induced changes such as pumping or irrigation. The rationale for identifying sites for the reconstituted CBR water-level monitoring network is to focus on locations in relatively undeveloped parts of aquifer systems where water-level fluctuations primarily reflect climatic variation, not groundwater withdrawals or artificial recharge. The areas where such sites are most likely to exist are in outcrop or poorly confined areas of major aquifer systems where recharge from precipitation or streams is the principal inflow.

Each District has a vast pool of knowledge and detailed information about existing observation wells in their State(s). The Office of Groundwater plans to utilize each District's expertise, and with assistance from Regional Ground- Water Specialists, identify a network of wells that primarily respond to climatic changes and to which CBR funds will be dedicated.

Funding for water-level monitoring from the CBR program in FY 96 was about $300,000 after accounting for monitoring in support of compacts and other Interior agencies. Uniform allocation by District is not appropriate from a hydrologic perspective, and yet it is desirable to have some components of a national network in every District. The consensus of the Ground-Water- Level Monitoring Task Group was that funding for individual Districts might range from $3,000 to $12,000. Thus, we are requesting that Districts provide a proposed set of wells at each of three possible funding levels: low--$3,000, medium-- $6,000, and high--$12,000.

Costs of network operations that need to be factored into the well selection process are frequency of water-level measurement, well maintenance, and ongoing quality assurance of the water- level measurements. For measurement of water levels to be funded under the CBR program, a frequency of at least one measurement every two months is required. More frequent measurements are desirable provided they would not result in significantly fewer wells in the network.

Guidelines for Well Selection

These guidelines are to assist in identifying potential index wells to be part of a national, long-term water-level monitoring network. The end product should be a sparse network of index wells where uninterrupted, long-term water-level records are collected. The water-level changes should primarily reflect climatic variability and not human influences. The climate variations of interest are those that affect recharge on monthly and longer time scales; not barometric or tidal influences.

Areas where water-level changes are relatively unaffected by human activity should be delineated on maps of the major aquifers in the District. Maps from the National Ground Water Atlas should be used in order to maintain regional continuity. All observation wells (current or discontinued) in the delineated areas should be considered for inclusion in the network if they satisfy the following required minimum criteria:

• Open to a single, known hydrogeologic unit
• Known well construction that allows good water-level measurements
• Located in unconfined aquifers or near-surface confined aquifers that respond to climatic fluctuations
• Minimally affected by pumpage and likely to remain so
• Essentially unaffected by irrigation, canals, and other potential sources of artificial recharge
• Long-term accessibility
• Well has never gone dry (not susceptible to going dry)

Additional desired characteristics:

• Representative of broad area (e.g., a regional aquifer)
• Complete characterization of the site is available
• A long record of water-level measurements exists
• Lithologic and geophysical logs available
• Alternative well identified for each site

Simple tables listing the potential wells identified for inclusion in the CBR network should be prepared for each incremental level of funding. The first table should include wells that would be measured at a low level of funding ($3,000 or less) (those judged to be of highest priority by the District). A second table should identify those wells that would be added (or frequency of measurement increased) at a medium level of funding ($6,000), and a third table should identify those wells and measurements that would be added at a high level of funding ($12,000).

For each well, the tables must include:

• Location--State, and latitude-longitude
• Local well ID or name
• Period of record (with frequency of measurement for period given; several periods may be necessary to characterize the record)
• Hydrograph available (Y=yes, N=no)
• Where is the record from this well (in GWSI? no previous record? in cooperator files? in project reports?)
• Depth of perforations and aquifer tapped
• Comments (e.g., include reason for discontinued measurements)

The final tables, which can be retrieved from ftp 130.11.51.175 at /var/ftp/CBR.Table.doc, should be transmitted via ftp as FrameMaker files to Velvie Stockdale (vestockd) at the Office of Groundwater. Also transmit to the Office of Groundwater and to the Regional Groundwater Specialist maps showing the well locations; hydrographs showing the period of record for each well (or recent site 5- to 10-year record); and, ideally, a precipitation hydrograph covering the same period. If a precipitation hydrograph is not available, include the name or identification of the most relevant precipitation station.

Hydroclimatic Data Base

In addition to defining a CBR network, we also would like to begin developing an historical hydroclimatic data base similar to that available for streamflow records (U.S. Geological Survey Open-File Report 92-129, by Slack and Landwehr). Thus, a fourth table should be prepared that identifies wells with historical water-level records that are not included in tables 1-3 but that meet (or met) the criteria set out above for the CBR network and have at least 5 years of record minimally affected by human activity. Such wells could include candidate wells that are of lower priority than the wells proposed for inclusion in the CBR network; wells that have been lost, abandoned, discontinued, or (subsequently) influenced by human activities; and wells that are not USGS accessible or measured. The current status of the records and the wells should be listed along with the location, depth, aquifer, and period of record. We will follow up on quality-assurance and record-acquisition issues for selected of these wells.

Implementation

Discussions or questions about any aspect of this well selection process can be addressed to Jilann Brunett (jbrunett) at (703) 648-5008, e.g. before the actual work of delineating areas of potential well locations on a map, and/or after wells have been identified in all tables for at least one aquifer. In particular, contact Jilann Brunett with any questions about obtaining copies of the National Ground Water Atlas maps, if not available.

Summary

In summary, it is the desire of the Office of Groundwater to develop a sound scientific basis for collecting water-level data for a sparse nationwide network of high-quality indicator or index wells. Initially, with the limited resources available, the focus is on wells that monitor natural hydrologic variability. As part of this effort, we also are beginning to develop an historical data base of water levels relatively unaffected by human activities.