Wisconsin Administrative Code › Department of Natural Resources › NR 100-199 - Environmental Protection General › Chapter NR 141 - Groundwater Monitoring Well Requirements › Section NR 141.11 - Filter packs

Current through June 24, 2024
All permanent groundwater monitoring wells installed in
				  unconsolidated material and used for the collection of water quality samples
				  shall be constructed with filter packs. Permanent groundwater monitoring wells
				  installed in bedrock may be constructed with filter packs. When used, the
				  filter pack shall be the only material in contact with the well screen. The
				  estimated and actual volume of filter pack material used shall be calculated
				  and reported to the department. All commercially prepared filter packs
				  installed in permanent groundwater monitoring wells shall meet the requirements
				  in sub. (1). All other filter packs shall meet the requirements in sub. (3).
(1) SPECIFICATIONS. The filter pack
				shall be a well sorted, silica based sand or gravel. The sand or gravel used
				for filter packs shall be hard and durable and shall have an average specific
				gravity of not less than 2.50. The sand and gravel shall be visibly free of
				clay, dust and micaceous and organic matter. Not more than 5% of the sand or
				gravel shall be soluble in a 10% hydrochloric acid solution. Thin, flat or
				elongated pieces of gravel, the maximum dimension of which exceeds 3 times the
				minimum dimension, may not constitute more than 2% of the material by weight.
				The filter pack for wells installed in unconsolidated material shall be sized
				to retain at least 50% of the surrounding formation based on a sieve analysis.
				In formations which are predominantly silt and clay, the filter pack shall be a
				fine sand. In bedrock, the filter pack shall be a medium or coarse sand or
				gravel. Crushed limestone, dolomite or any material containing clay or any
				other material that will adversely impact on the performance of the monitoring
				well may not be used as filter pack.
(2) INSTALLATION. The filter pack shall
				extend from 6 inches beneath the bottom of the well to 2 feet above the top of
				the well screen. For water table observation wells constructed in areas where
				the depth to water table is less than 7 feet, the required filter pack height
				above the top of the well screen may be reduced to 6 inches to allow for the
				required amount of annular space sealant to be placed. To ensure that the
				filter pack is installed evenly surrounding the well screen and casing over the
				proper depth interval, a tape measure, measuring rod or similar device shall be
				used to measure the height of the filter pack. The tape measure, measuring rod
				or similar device shall be carefully raised and lowered while the filter pack
				is being installed to identify bridging. If bridging occurs the filter pack
				material shall be tamped into place, surrounding the well screen and casing,
				using a measuring rod or similar device.
(3) COLLAPSED FORMATION. Collapsed formation
				may be used as filter pack material if the collapsed formation will limit the
				passage of formation fines into the well screen and either an artificial filter
				pack cannot be installed or the formation grain size is greater than or equal
				to fine sand sized grains. The grain size distribution of the collapsed
				formation shall be such that at least 90% of the formation will be retained by
				the well screen based on a sieve analysis. Analysis of the collapsed formation
				for specific gravity and particle size shall be performed and documentation
				shall be submitted to the department to support its use as an acceptable filter
				pack. Following review of the submitted information, the department may require
				new well construction if the collapsed formation analysis is not consistent
				with the filter pack specifications.