Probable High Groundwater Levels

(1) For areas in Massachusetts other than Cape Cod: (a) Sand and gravel areas. For estimating the probable high groundwater levels in sand and gravel areas in Massachusetts except Cape Cod, the following formula shall be used wherever practicable. Use of this formula allows for the estimation of the potential groundwater level rise at the facility site by correlation with the potential rise in an off-site observation well if the climatic trends and hydrogeologic conditions at the site and the well are similar. For an in-depth discussion of the derivation of this formula and its use see: Probable High Groundwater Levels in Massachusetts, U.S. Geological Survey, Water Resources Investigations, Open-File Report 80-1205 by Michael H. Frimpter. Sh = Sc - Sr (OWc - OWmax) OWr where: Sh = estimated depth to probable high water level at the site; Sc = measured depth to water at the site (ground level to water table); OWc = measured depth to water in the observation well which is used to correlate with the water levels at the site (ground level to water table); OWmax = depth to recorded maximum water level at the observation well which is used to correlate with the water levels at the site; Sr = range of water level where the site is located; and OWr = recorded maximum value of annual range of water level at the observation well which is used to correlate with the water levels at the site. In the above equation, Sc and OWc shall be measured in the same month. The observation well which is chosen to correlate with the water levels at the facility site shall be among those included in the report Groundwater Levels in Massachusetts, 1936-74: U.S. Geological Survey Open-File report, Massachusetts Hydrologic-Data Report 17 by Anthony Maevsky. The observation well shall be located in the same type of climate and hydrogeologic environment as the facility site. Values of OWmax and OWr shall be obtained from the chosen observation-well record. OWmax shall be found by reviewing the historical record of measurements and finding the depth from ground level to water table level that corresponds to the maximum water level ever recorded at the observation well site. OWr shall be determined by finding the maximum water level fluctuation that ever occurred during one year of recordings at the observation well (i.e., the "range" or the maximum difference in water table levels ever recorded during a one year period). The value of Sr shall be as follows: Sand and gravel deposits on terraces and hillsides 10 feet Sand and gravel deposits in valleys 4 feet These values represent a range of water level at the site that is unlikely to be exceeded. (b) Areas of till. For estimating the probable high groundwater level in areas of till, a series of measurements shall be taken to determine the high groundwater level for a particular year, and the following method shall be used, wherever practicable. To estimate the maximum water level, a proportion is used. That proportion is: The potential water-level rise at the site is to the maximum annual water-level range at the site as the potential water-level rise at an observation well is to the maximum annual water-level range at the observation well, where the potential rise is the difference between the highest and current water levels. For the use of the formula below, a value of five feet was chosen to represent the maximum annual water-level range at the site. Analysis of records of over 5000 water level measurements in 15 wells in till between 1936 and 1982 show that the mean range of maximum annual water levels is about five feet. For an illustration of the variables which are used in the formula, see Figure 30.675(1)(b). Difference between current Difference between current and and maximum levels at site = maximum levels at observation well Range of maximum annual Range of maximum annual level at level at site observation well Sc - Sm = OWc - OWm 5 OWr OWc - OWm Sm = Sc - 5 x OWr Steps in the method are as follows: Measure and record the water level on a weekly basis from March 15 to May 15. From the observation well location map select the most representative observation well on a basis of geographic proximity. From reports of the U.S. Geological Survey determine the water level (OWc) in that observation well for the concurrent date, the date closest to the date at which the highest water level was measured at the site. Determine from Table 30.675(1)(b), provided here, the maximum recorded water level (OWm) and the range of annual maximum water level (OWr) for the same observation well. Substitute these values of OWc, OWm and OWr, and the current maximum annual water level measured at the site (Sc) in the formula and solve for the estimated maximum water level at the site (Sm). Based on the small available sample of 10 representative observation wells in till there is a 10% chance that the maximum annual range at a site would exceed the estimate by about one foot. See the range of OWr in Table 30.675(1)(b). TABLE 30.675(1)(b) S)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))Q Maximum Water Levels and Maimum Annual Water-Level Ranges for Observation Wells in Till S)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))Q LOCATION NUMBER OWm OWr Andover AJW-26 Cheshire CJW-2 East Bridgewater EBW-30 Great Barrington GMW-2 Lowell L2W-14 Middleborough MTW-82 Northborough NUW-38 Topsfield TQW-1 Weymouth XGW-2 Winchester XOW-14 S)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))Q Most, but not all, facility sites will be located in one of the hydrogeologic settings discussed above. As an example, a surface impoundment might be located on a terrace composed largely of silt. In such a case, the formula is not valid. The estimation of the probable high groundwater levels shall be based upon measurements of the water level at the facility site in the months of March, April and May and a best possible prediction of the highest water table levels that can reasonably be expected throughout the facility's operating life (and beyond if a land disposal facility). Soil mottling may help to verify this prediction. In rare instances, long term historical data (15 years or more) of water levels may actually be available at the facility site. In these cases, the probable high groundwater levels can be determined directly from this data. (2) Probable High Groundwater Levels For Cape Cod. For estimating the probable high water level in sand and gravel areas of Cape Cod, the following method shall be used wherever practicable. It cannot be applied in areas where low permeability layers of silt, clay or till are present. Some parts of Cape Cod show no pattern for annual water level range. In these locations, soil conditions are unsuitable for water level estimation. In cases where the following procedure is not applicable, tests for water levels shall be conducted at the site during March, April and May and a best possible judgment made of the probable high groundwater levels considering hydro-geologic and climatic trends. Probable high groundwater levels at a site in Cape Cod shall be made by correlation of a single water-level measurement from a facility test site with water level records from one of nine index wells which have been established bythe U.S. Geological Survey. For the rationale behind the establishment of the estimating procedure, and for an in-depth discussion of its use, see: Probable High Groundwater Levels on Cape Cod, Massachusetts, U.S. Geological Survey, Water-Resources Investigations, Open-File Report 80-1008 by Michael H. Frimpter. Also, in order to estimate the probable high groundwater levels, Plate 1 and Plate 2 from that report will need to be utilized. These plates subdivide Cape Cod into nine areas in which water level fluctuations in each area are best represented by a particular index well. The formula which is used to estimate the depth to the probable high water level at the site is similar to the formula that is used for finding probable high water levels for areas in Massachusetts other than Cape Cod: Sh = Sc - Sr (OWc - OWmax) OWr or Estimated depth to probable = measured depth to - water level high water table water at the site adjustment where Sr(OWc - OWmax) = the water level adjustment OWr However, in this case Sr, the maximum value of the annual range of water level at the facility site, will be one of five values (2, 3, 4, 5 or 6 feet), depending upon the particular geographical zone in which the facility is situated. In areas of perched groundwater tables, a value Sr = 10 may be used, but only with written approval of the Department. In order to simplify the use of this formula, a series of nine tables has been developed. These tables appear in the Report cited in 310 CMR 30.675(2). One table has been prepared for each of the nine index observation wells. The water level adjustment value shall be found from these tables once the geographic zone of the facility site is known and the water levels in the index well and at a specific site have been measured. Procedure 1. Measure, to the nearest 1/10th of a foot, the depth to the water table below land surface at the facility site. 2. Find the location on Plate 1 or Plate 2 of the facility test site. From this Plate determine the geographical zone in which the facility site exists (Zone A, B, C, D or E). Also determine the index well which shall be used. 3. Determine the depth to water in the appropriate index well for the month in which the depth to water was measured at the facility site (or previous month if current data are not yet available; see the NOTE below). (The U.S. Geological survey reports the index-well measurements monthly to the Regional Environmental Engineers of the Department and to the Cape Cod Planning and Economic Development Commission.) 4. Use the appropriate table (See Sample Table 30.675(2)) and find the depth to water level adjustment value from the Table based upon the geographical zone where the site is located and the reported depth to water in the index well. (Refer to the U.S.G.S. report for the complete set of tables). 5. Subtract the water-level adjustment value from the measured depth to water at the facilitysite to obtain the estimate of depth to the probable high water level. NOTE: Because the locations of the boundaries between the areas represented by the index wells are somewhat inexact, the above-cited report suggests that, when the site being evaluated is within 1,000 feet of such a boundary, estimates should be calculated from both index wells. The higher of the two groundwater levels calculated would be less likely to be exceeded. The water level for the month in which the site testing was done should be used in Procedure 3. However, the water level reported for the index well for the month previous to the month in which the site was tested may be used, provided that 0.25 feet is subtracted from the depth to the water level for each week or fraction thereof between the date of the site test and the end of the previous month. For example, if an estimate for a site test made on the 8th of August (8/7 = 1.14) should be adjusted by subtracting 0.3 feet from the reported July water level for the index well. This adjustment need only be applied when the test site is measured in the months of May, June, July, August, September, or October. The adjustment is based on the recorded maximum water level decline of 0.99 feet over a one month period in all nine index wells and the observation that groundwater levels on Cape Cod generally decline from May through October. SAMPLE TABLE 30.675(2) Water-level Adjustments, in Feet, for Use with Index Well Barnstable A1W-230 (Located in Zone C) S)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))Q Measured water level (OWc in Zone A Zone B Zone C Zone D Zone E feet below land surface) (2/4)1 (3/4)1 (4/4)1 (5/4)1 (6/4)1 S)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))Q .0 .1 .1 .1 .1 .1 .1 .2 .2 .3 .1 .2 .3 .4 .4 .2 .3 .4 .5 .6 .2 .4 .5 .6 .7 .3 .4 .6 .7 .9 .3 .5 .7 .9 .4 .6 .8 .4 .7 .9 .5 .7 .5 .8 .6 .9 .6 .7 .7 .8 .8 .9 .9 SAMPLE TABLE 30.675(2) (continued) Water-level Adjustments, in Feet, for Use with Index Well Barnstable A1W-230 (Located in Zone C) S)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))Q Measured water level (OWc in Zone A Zone B Zone C Zone D Zone E feet below land surface) (2/4)1 (3/4)1 (4/4)1 (5/4)1 (6/4)1 S)))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))Q )))))))))))))))))Q 1 Sr /OWr = 2 Recorded highest water level (OWmax). (3) Alternative Methods. The Department may accept alternative methods for determining probable high groundwater levels if such methods are demonstrated to give equally reliable results. For the purposes of 310 CMR 30.000, alternative methods may be used only following written approval by the Department.