OAR 690-240-0475: Well Seals

(1) Proper seal placement is essential to minimize the potential for movement of water and contaminants from the surface, or other waterbearing zones, into the monitoring well. Appropriate sealing materials may vary depending on the site characteristics and the substances being monitored. Well seals shall consist of a physically and chemically stable hydrated grout slurry composed of: (a) Neat cement; or (b) Sodium bentonite; or (c) A cement-bentonite grout mixture containing no more than five percent bentonite by dry weight; or (d) Sodium bentonite granules, pellets or chips placed in an unhydrated state, and subsequently hydrated downhole. (2) Grout slurries shall be mixed in the proper proportions and placed in the bore hole in such a manner as to prevent excessive shrinkage, water loss, chemical breakdown, bridging or invasion into the filter pack. The following procedures shall be utilized if a grout slurry is to be the sealing material: (a) Neat cement used for grouting shall be American Petroleum Institute Class A or B, or ASTM C-150 Type I or II neat cement with no additives, mixed in the proportion of 5.2 gallons of water per standard 94 pound sack and having a mud weight of approximately 15.6 pounds per gallon; (b) Bentonite used as a slurry for grouting shall be a high solids granular sodium bentonite mixed according to the manufacturer's directions, having a minimum mud weight of at least 9.5 pounds per gallon, and containing at least 20 percent solids. Mixing methods should be used which prevent the slurry from being excessively lumpy; (c) When a mixture of cement and bentonite is used as a slurry for grouting, the cement shall be American Petroleum Institute Class A or B, or ASTM C-150 Type I or II neat cement. The slurry shall be no more than five percent, by dry weight, of sodium bentonite gel powder (3.75 pounds of bentonite per sack of cement). For each pound of bentonite added, up to an additional 0.7 gallons of water shall be added to the original neat cement mix. The water and bentonite shall be mixed first, and then the cement added to the bentonite slurry. The cement-bentonite mixture shall have a mud weight of approximately 14.1 pounds per gallon; (d) Prior to placing grout in the annular space, the grout slurry weight shall be measured by ASTM Test Method D-4380-84. Grout slurry shall not be placed in the annular space until the grout slurry weight is within ten percent of the weight specified in subsection (2)(a), (b) or (c) of this rule; (e) Grout slurries shall be placed from the bottom of the annular space upward in such a manner as to completely fill the sealing interval. Grout slurries shall begin at the top of the filter pack seal overlying the filter pack and extend to the bottom of the surface seal. If the grout slurry is intended to serve as the surface seal, it shall extend to land surface; (f) Grout slurries shall be placed through a side discharge grout pipe by gravity flow or by pumping to ensure positive placement without bridging or wash-out of previously placed annular materials. The discharge end of the grout pipe shall remain submerged in the grout throughout the sealing operation; (g) Prior to discontinuing placement of grout in the annular space, grout slurry returns from the annular space shall be measured by ASTM Test Method D-4380-84. Placement of grout slurry in the annular space shall continue until the returns are within ten percent of the weight specified in subsection (2)(a), (b) or (c) of this rule. (3) Bentonite used in an unhydrated form shall be sodium bentonite granules, pellets or chips. Unhydrated bentonite shall be specifically designed for sealing wells and be within industry tolerances for dry western sodium bentonite. Bentonite shall be free of polymers that promote bacterial growth. The following procedures shall be adhered to if dry bentonite is used for sealing: (a) Dry, poured bentonite seals shall only be used if the depth to the bottom of the seal is less than fifty feet and the standing water column in the bore hole or annular space is less than twenty-five feet deep at the time of seal placement. Only sodium bentonite chips manufactured to be greater than 1/4 inch or tablets shall be used below the water level in the sealing interval; (b) Pour rate shall be three minutes or slower per 50 pound sack in the water-filled portion of the annulus; (c) A sounding or tamping tool shall be used in the bore hole or annular space during pouring to measure fill rate and to break up possible bridges or cake formation; (d) Care shall be taken to minimize the introduction of bentonite dust into the sealing interval; (e) In a dry sealing interval, bentonite shall be hydrated with potable water in two foot lifts to ensure activation. (4) The estimated and actual volume of sealing material used shall be calculated and reported to the Department. [Publications referenced are available from the agency.]