OAR 690-210-0340: Method of Placement of Unhydrated Bentonite

(1) An upper oversize drillhole, four inches greater than the nominal inside diameter of the permanent well casing, shall be constructed to a minimum depth of 18 feet below land surface. The use of unhydrated bentonite as a casing seal shall not be allowed deeper than 200 feet below land surface. In the event that the materials penetrated by the oversize drillhole cave, or tend to cave, an outer temporary surface casing shall be used to case out the caving materials during construction of the oversize drillhole. The outer temporary surface casing shall be removed during seal installation and before completion of the well. (2) In the event groundwater is encountered during the construction of the oversize drillhole, only unhydrated bentonite chips manufactured to be 3/8 inch to 3/4 inch, pellets or tablets shall be allowed in the water-filled portion of the annulus. A maximum of 50 feet of water may be present in the sealing interval. Unhydrated bentonite shall be screened across a minimum 1/4 inch mesh screen prior to being placed in the water-filled portion of the annulus to minimize the introduction of bentonite dust into the seal interval. Unhydrated bentonite shall not be used: (a) In the water-filled portion of a temporary casing; or (b) If there is any uphole flow in the annular seal interval. (3) Unhydrated bentonite may only be used as an annular seal material below the water level in a well when the groundwater it comes in contact with does not exceed 800 parts per million (ppm) total dissolved solids (TDS). (a) Unhydrated bentonite may be used as an annular seal material in water supply wells exceeding 800 ppm TDS if the bentonite manufacturer provides documentation that their product can be used in water that exceeds 800 ppm TDS. (A) Prior Department approval is required before placement. (B) The bentonite manufacturer’s documentation and Department approval shall be submitted with the Water Supply Well Report as required in OAR 690-205-0210. (b) In all cases, the TDS shall be reported on the Water Supply Well Report as required in OAR 690-205-0210. (c) Regardless of the reported TDS, the quality of the water in the well shall not interfere with the proper hydration of bentonite. (4) After placement of the permanent casing, the annular space shall be filled to land surface with bentonite. The annular space shall be kept full of bentonite to land surface while drilling or driving casing. A calibrated sounding tape with weight shall be used continuously in the sealing interval during bentonite placement to measure fill rate and to check for and break up possible bridges. (5) Placement of bentonite shall conform to the manufacturer's specifications and result in a seal that is free of voids or bridges. Care shall be taken to minimize the introduction of bentonite dust into the sealing interval. (6) The volume of sealing material required shall be calculated prior to seal installation. The calculated volume and actual volume used shall be reported on the water supply well report. (7) Unhydrated bentonite chip, pellet or tablet annular seals shall be hydrated from land surface with potable water prior to removing the drilling machine from the well site. The hydration shall begin once all of the bentonite annular seal material has been placed and shall end when the annular seal interval refuses to take more water or after at least one annular space volume of water has been placed. (8) Granular bentonite may only be used as an annular seal material in a dry annular space above the interval where water was first encountered. Granular bentonite shall not be screened or hydrated during placement. (9) Pour rate shall be two minutes or slower per 50 pound sack in the water-filled portion of the annulus.