A Technique for Estimating Ground-Water Levels at sites in Rhode Island from Observation-Well DataWater-Resources Investigations Report 94-4138 |
Estimates of future high, median, and low ground-water levels in Rhode Island are needed for engineering and architectural design decisions and for appropriate selection of land uses. For example, the failure of individual underground sewage-disposal systems due to high ground-water levels can be avoided if accurate water-level estimates are available. Estimates of extreme or average conditions are needed because short-duration re-construction observations are unlikely to be representative.
The technique described in this report utilizes a single water-level measurement at a site of interest, in combination with a long-term water-level record at an observation well, to estimate the long-term high, median, and low water levels at the site of interest. The transfer of information to the site of interest depends on four fundamental assumptions: (1) Water levels will fluctuate in the future as they have in the past, (2) Water levels fluctuate seasonally, (3) Ground-water fluctuations depend on site geology, and (4) Water levels throughout Rhode Island are affected by similar precipitation and climate. The technique is based on the equivalent relation between the ratio of potential water-level change to maximum annual water-level range at the site and the ratio of potential water-level change to annual water-level range at the observation well. Equations for estimating high, median, and low water levels, and graphs of probable annual water-level range are given for selecting representative ranges of water levels for sand and gravel and till in Rhode Island.
The accuracy of the technique is evaluated by use of the equations to estimate water levels at long-term observation wells where high, median, and low water levels are known from monthly measurements over many years. As a test of the estimating procedure, 6,697 estimates each of high, median, and low water levels (depth to water level exceeded 95, 50, and 5 percent of the time, respectively) were compared with measured water levels exceeded 95, 50, and 5 percent of the time at 14 sites unaffected by pumping or other known factors. Mean squared errors (average differences squared, between estimated and measured water levels) for the estimates ranged from 0.34 to 1.53 ft2 for high water levels, 0.30 to 1.22 ft2 for median water levels, and 0.32 to 2.55 ft2 for low water levels. All mean squared errors are less than the State required 3-foot separation between the bottom of the stone underlying the seepage system and the maximum altitude of the water table. This degree of accuracy is acceptable for many design purposes.
Abstract
Introduction
Purpose and Scope
Approach
Hydrogeologic Setting
Observation-Well Network;
Fluctuations of Ground-Water Levels
Range
Frequency Distribution
Technique for Estimating Ground-Water Levels from Observation-Well Data
Estimation Equations
Selection of Index Wells
Guidelines for Selecting Index Wells to Estimate Water Levels at Sites in Sand and Gravel
Guidelines for Selecting Index Wells to Estimate Water Levels at Sites in Till
Procedure for Use of the Technique
Example Use of the Technique for Estimating Depth to High Ground-Water Level
Limitations and Special Conditions of Estimation Technique
Applicability of Estimation Technique to Areas Outside Rhode Island
Summary
References Cited
Appendix A. Correlation and Regression Analysis of Ground-Water Levels
Appendix B. Analysis of Observation Wells Used as Index Wells
1. Diagram showing normal functioning septic system and septic-system failure caused by unanticipated high ground-water levels.
2. Map showing locations of observation wells completed in sand and gravel in Rhode Island and observation wells completed in sand and gravel and in till in parts of Massachusetts, 1991.
3. Map showing locations of observation wells completed in till in Rhode Island, 1946-92.
4-13. Graphs showing:
4. Water-level fluctuations in observation well BRW-23, Rhode Island, caused by tides, August 24 to September 1, 1977.
5. Seasonal water-level fluctuations in observation well RIW-417, estimated potential evapotranspiration, and monthly precipitation at Kingston, Rhode Island, 1987-89.
6. Water-level fluctuations in an observation well caused by pumping of nearby wells.
7. Water levels in observation wells EXW-6 and PRW-105 1, Rhode Island, 1948-90.
8. Probability of water-level ranges for wells completed in sand and gravel in Rhode Island, 1946-90.
9. Probability of water-level ranges for wells completed in till in Rhode Island, 1946-61.
10. Frequency of the highest and lowest measured annual water levels, by month, in 21 observation wells completed in sand and gravel in Rhode Island.
11. Frequency of the highest and lowest measured annual water levels, by month, in 19 observation wells completed in till in Rhode Island.
12. Monthly water-level fluctuations during 1986-89 and historical (1954-90) water-level frequency in well NKW-255, Rhode Island.
13. Difference between smallest measured depth to water on record and depth to water exceeded 95 percent of the time in wells completed in sand and gravel in Rhode Island.
14. Diagram showing water-level measurements at observation well and test site as related to estimation equations, Rhode Island.
15,16. Plots showing:
15. Relation of water levels in two wells (WEW-522 and CHW-18) completed in sand and gravel in Rhode Island.
16. Relation of water levels in two wells (RIW-231 and WGW-181) completed in sand and gravel in Rhode Island.
17, 18. Graphs showing:
17. Measured and estimated water levels in a well completed in till (EXW-332) and measured water levels in a well completed in sand and gravel (NKW-255), Rhode Island, 1956-59.
18. Measured water levels in well RIW-417 and measured and estimated water levels for periods of missing record in well SNW-5 15, Rhode Island, 1980-83.
1. Town codes for wells in Rhode Island and parts of Massachusetts
2. Hydrogeologic characteristics of observation wells completed in sand and gravel in Rhode Island
3. Hydrogeologic characteristics of observation wells completed in till in Rhode Island
4. Frequency distribution of monthly ground-water levels for wells completed in sand and gravel in Rhode Island
5. Frequency distribution of ground-water levels computed from 5-day and monthly measurements for four wells in Rhode Island
6. Correlation of water levels in wells completed in sand and gravel in Rhode Island, 1946-90
7. Correlation of water levels in selected wells completed in till in Rhode Island, 1946-61
8. Correlation of water levels in selected wells completed in sand and gravel with water levels in selected wells completed in till in Rhode Island, 1946-61
9. Correlation of water levels in selected wells completed in sand and gravel in Massachusetts with those in selected wells completed in sand and gravel in Rhode Island, 1946-90
10. Mean squared error for estimates of high water levels for wells in Rhode Island completed in sand and gravel, based on water levels in index well
11. Mean squared error for estimates of median water levels for wells in Rhode Island completed in sand and gravel based on water levels in index well
12. Mean squared error for estimates of low water levels for wells in Rhode Island completed in sand and gravel, based on water levels in index well
13. Average mean squared error for wells in Rhode Island completed in sand and gravel used as index wells in equations 3, 4, and 5 to estimate depth to water level exceeded 95, 50, and 5 percent of the time in all other Rhode Island wells completed in sand and gravel
14. Mean squared error and mean of 6,697 estimates of high, median, and low water levels in wells in Rhode Island, based on water levels in well CHW-18 (1946-90) and equations 3, 4, and 5
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The citation for this report, in USGS format, is as follows:
Socolow, R.S, Frimpter, M.H., Turtora, M., and Bell, R.W., 1994, A technique for estimating ground-water levels at sites in Rhode Island from observation-well data: U.S. Geological Survey Water-Resources Investigations Report 94-4138, 43 p.
For more information about USGS activities in Massachusetts-Rhode Island District, visit the USGS Massachusetts-Rhode Island Home Page.
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