Use of Vertical Gradient to Compensate for Disparate Completion Depths When Characterizing Horizontal Flow Direction in Stacked Fractured Aquifers During an

MTBE Investigation, Mariposa County, California

William E. Ackland and Herman Schymiczek, HerSchy Environmental, Inc. Abstract During shallow environmental investigations in unconsolidated sediments, many wells at an individual site are completed to similar depths. This thereby negates the influence of vertical gradient in any three-dimensional flow regime. However, the nature of aquifers in fractured granitic bedrock requires drilling until a water producing fracture is encountered, not a specific target depth. Such is the case at the subject site in Mariposa County, California. MTBE contamination at this site has migrated from the unconfined aquifer in relatively shallow soil and decomposed granite into a shallow and deep bedrock aquifer. The resultant depth discrepancies in monitoring wells installed into the latter two aquifers rendered potentiometric maps at this site invalid for evaluating groundwater flow directions due to the high vertical gradient in the stacked bedrock aquifers. Nested completion wells were planned and the boreholes drilled for the two bedrock aquifers to allow the calculation of vertical gradients. Multiple completion intervals were chosen from each borehole using wireline logging utilizing, among other tools, a video camera and optical televiewer. The optical televiewer yielded a compass oriented, 360 degree “unwrapped” full-length still picture of the borehole. This data was used to target specific fractures for depth discrete groundwater sampling prior to installation of well casing. Analytical results and hole diameter constraints further narrowed the possible completions to a maximum of three per borehole. After well installation, subsequent water level monitoring of the nested wells allowed vertical gradients to be calculated for each aquifer. The mathematically derived vertical gradients were then applied to “correct” the observed static water elevation of wells in both aquifers to conform to a uniform theoretical completion depth instead of the actual completion depths. Water table maps drafted from these “corrected” elevations yielded contours devoid of the sharper curves and differing orientations common to the “uncorrected” maps. In order to more fully delineate the MTBE plume, subsequent wells were planned and installed based on the “corrected” data. The new downgradient well location chosen by this method has been found to contain MTBE in higher concentrations than any of the lateral wells. Judging by the presence or absence of the MTBE contaminant, this correction process allowed accurate prediction of the primary flow and transport direction at the investigation site. Introduction: During the course of a groundwater contamination investigation at the subject site in Mariposa County, California, the fuel oxygenate methyl tertiary butyl ether (MTBE) was discovered in multiple nearby domestic wells. No municipal water supply exists in the area, and domestic wells are abundant, although completion details for many of these wells were often sketchy or nonexistent. Primary transport direction needed to be understood to better ascertain the extent of the problem, identify impacted and at-risk domestic wells, and to provide useful locations for future remediation and monitoring wells. A site map showing all wells as of the time of this writing is shown in Figure 1.

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MW-11D

MW-18D

MW-17D

CARDLOCK STATION

COUNTY YARD

Fence Line

Property Line

Property Line

Figure 1 – Site Map. Initial investigations showed an unconfined aquifer in soil and decomposed granite extending down to the unweathered granite interface generally about 30 feet below ground surface (BGS). Head levels in deeper monitoring wells revealed the presence of a shallow bedrock aquifer, while levels in nearby domestic wells indicated an additional, deeper aquifer.

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Standing water level in the unconfined aquifer is generally about 15 feet BGS. The shallow bedrock aquifer (also called the intermediate aquifer) was initially confused with the unconfined aquifer due to the steep gradient present on-site and static water levels only three to six feet lower. Water levels in the deep aquifer stand between 60 to 70 feet BGS. Water producing fractures are found between 30 and 85 feet BGS for the intermediate, and 105 to 250 feet (deepest known) BGS for the deep aquifer. Even after identification of the intermediate aquifer, no sensible potentiometric maps for this aquifer could be drafted, as water table elevations did not conform to any reasonable surface. However, completion depths for wells in this aquifer varied by about 45 feet. Sometimes, the vertical discrepancy between completion depths was greater than the horizontal distance between wells. Since the granitic matrix housing the fractures is impermeable, drilling could not be terminated at a specific target depth but had to continue until a water producing fracture was encountered. As groundwater flow is three-dimensional in nature, it was considered likely water level elevations in the monitoring wells were reflective of vertical as well as horizontal gradients. Typically, a shallow environmental investigation in unconsolidated sediments may encompass many wells completed to similar depths. This thereby negates the influence of vertical gradient in any three-dimensional flow regime. However, the radical difference in completion depths at this site meant vertical gradients could not be discounted. Therefore, in order to evaluate the horizontal vector of flow, the influence of vertical gradient needed to be taken into account. For the multiple completion monitoring wells at this site, the letter following the number denotes the relative completion depth in that particular boring. For example, monitoring well MW-13 contains three separate wells designated as MW-13S (shallowest), MW-13I (intermediate), and MW-13D (deepest). This does not mean the MW-13I completion is in the intermediate aquifer, simply that it is the intermediate depth completion for that nested well. In this particular instance, MW-13S belongs to the intermediate aquifer, while MW-13I and MW-13D are both in the deep aquifer. For ease of reference, the following table is included:

Table 1 - Well Completions and Screened Depths (BGS) by Aquifer Shallow Aquifer Intermediate Aquifer Deep Aquifer

EX-1 5-35 MW-8 69.5-89.5 EX-2 150-180 MW-1 15-35 MW-9 40-60 MW-7 140-150 MW-2 9.5-29.5 MW-10S 49-54 MW-10I 96.5-126.5 MW-3 5-25 MW-11S 26-31 MW-10D 165.5-175.5 MW-4 8-28 MW-11D 36-41 MW-12D 118-128 MW-5 9.75-29.75 MW-13S 43-53 MW-13I 107-137 MW-6 10-30 MW-14S 68-78 MW-13D 147-152

MW-12S 33-48 MW-15S 37.5-47.5 MW-14D 152.5-172.5 MW-16 5- 5 3 MW-17S 28-38 MW-15I 136.5-156.5

MW-17D 41-43.5 MW 15D - 225.5 35.5 -2 MW-18S 26-43.5 MW-18D 47-57

Methods of Investigation: Drilling Procedures and Well Installation: Bedrock monitoring well installation utilized air rotary drilling equipment with a six and one-quarter inch diameter drill bit. After encountering unweathered bedrock, further advancement of each borehole was temporarily halted until six inch PVC conductor casing could be installed. Each boring was then advanced to a depth of approximately five feet below a water influx of about five gallons per minute (GPM) or more.

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Multiple logging techniques were used to determine the most beneficial fractures to screen. The data obtained from these logs enabled the targeting of specific fractures for depth discrete water sampling in the open borehole. Wells were then constructed through the PVC conductor casing. Schedule 40 PVC well casing with screw joints was used for all completions. The screened intervals in all monitoring wells utilized two-inch schedule 40 PVC with 0.020-inch factory slotting. Annular materials consist of number three sand, medium bentonite chips (regular and time release coated), and bentonite grout. Groundwater extraction well EX-2 was constructed with four-inch schedule 40 PVC and also utilized 0.020-inch factory slotted screen. Upon completion of well installation, wells were developed by purging with a submersible pump until the discharge was relatively clear and free of sand. Pre-existing and newly installed well elevations were subsequently surveyed to the nearest 0.01 feet using the NAVD 88 vertical datum, state plane coordinate system, zone 3. Downhole Logging: After completion of drilling, but before monitoring well installation, the wells were allowed to sit undisturbed for at least two days. This allowed sediment to settle out, and the water column to equalize, before conducting wireline downhole logging. These logs included video logging, optical televiewer with directional indicator, and FloVision. Borehole Optical Televiewer: The optical televiewer manufactured by Robertson Geologging Limited has a radial resolution of 720 pixels/360 degrees, and a vertical resolution of 1 millimeter. It yields a compass oriented, 360 degree “unwrapped” full-length still picture of the borehole. Planar fractures which intersect the open hole will appear as sinusoidal features. A measurement of amplitude, therefore, is a measurement of the vertical difference between the elevation where a fracture enters the well, and where it exits. The lowest point of the sine wave feature is where the fracture exits the well. This is also the dip direction of the fracture. Borehole diameter is known based on the drill bit used. So, the dip angle of the fracture may be calculated from the arcsine of the vertical difference divided by the borehole diameter. Video Log: Real time, downhole video logging was also conducted using a SideScan video survey camera. As the name suggests, this type of camera is capable of recording a video image from either the down-hole or side views. The video recording was then studied in conjunction with the optical televiewer log to determine which depth intervals should be sampled prior to well completion. FloVision: The FloVision tool is a hollow tube with plastic skirts around the base and top which channel all water flow in the well through the tube. A flapper valve in the tube is connected to an indicator needle. Downward flow through the tube causes the needle to deflect one way, while upward flow causes the needle to deflect the other direction. A video camera mounted above the tube allows the operator to observe the deflection and control the rate of descent of the FloVision instrument. The operator can adjust the rate of descent or ascent to match the rate of vertical flow in the well by observation of the needle deflections. Mechanical measurements of cable length played in or out compared with time yield vertical flow velocities. Pre-Completion Sampling: Data gathered from the optical televiewer and video logs were used to identify specific fractures of interest. Groundwater samples were subsequently gathered at these depths by slowly lowering a sealed sampler (evacuated by vacuum pump under 16 inches of mercury) to the specified depth and opening the aperture. The sampler was then retrieved and the sample collected in 40 milliliter bottles. In order to obtain as accurate a 625

sample as possible, these samples were not gathered on the same day as the optical televiewer or video surveys - in order to minimize artificial mixing of the water column. Inevitably, the number of fractures which could be bearing water with MTBE were greater than the number of two-inch completions possible in a 6.25 inch diameter hole. Based on this sampling, fractures yielding higher concentrations of MTBE could often be identified. The analytical results of this testing allowed the number of completions to be further narrowed to no more than three per borehole. Results of Investigation: In February of 2003, MW-10 through MW-12 were installed on the site. The producing fractures screened by MW-11 both belonged to the intermediate aquifer, while MW-10 contained one intermediate aquifer and two deep aquifer fractures. Well MW-12 contained one shallow aquifer completion and one deep aquifer completion. An additional deep aquifer completion southwest of the line defined by MW-10/MW-7/MW-12 was planned and approved by the state. However, while drilling MW-10 and MW-12, the third well was not allowed by the County due to concerns about a nearby septic system. With three wells in a line, flow direction could not be determined for the deep aquifer, and intermediate aquifer data was relied on for flow and transport direction in the fractured aquifers until additional deep monitoring points could be installed. Linear interpolation contouring of the raw elevation data for the intermediate aquifer gathered from the March 2003 quarterly monitoring (Figure 2) indicated groundwater in this aquifer flowed northwesterly. This orientation seemed suspicious, given the presence of Mariposa Creek drainage adjacent to the west and the San Joaquin Valley farther west-southwest.

MW-10S

MW-11D

1,740

.20

1,74

0.60

1,74

1.00

1,741.24

1,741.26

1,739.99

1,740.47

Flow Direction = N. 50 W.

Gradient = 0.016

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Figure 2 – Intermediate Aquifer: Uncorrected Data, March 2003.

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The noticeable difference in groundwater elevation in the nested wells of MW-10 and MW-11 suggested a significant vertical gradient present on-site. As some of the wells were separated by a larger distance vertically than horizontally, this meant vertical gradients may have been influencing water level differences between wells to a greater degree than horizontal gradient. Therefore, if measured water levels could be adjusted for relative completion depths by correcting for, or removing, the influence of vertical gradient, then the resulting levels should represent the true horizontal component of groundwater flow. Calculation and Application of Vertical Gradient Correction: Vertical gradient was calculated for the intermediate aquifer by dividing the difference in water table elevation between MW-11D and MW-11S by the vertical difference between the midpoint of the relative screens, or: Equation 1: Water Elevation (MW-11D) – Water Elevation (MW-11S)

Depth to screen midpoint (MW-11D) - Depth to screen midpoint (MW-11S) Only one nested well with two intermediate aquifer completions existed on-site to measure vertical gradient. Therefore, to correct all intermediate aquifer wells using this gradient, it was necessary to assume vertical gradient was uniform (horizontally and vertically) across the site. Also, as the site is relatively flat, differences in surface elevation were considered to be negligible. Then, an arbitrary uniform theoretical completion depth of 51.5 feet BGS (midpoint of screen for MW-10S) was chosen. To correct the water table elevations, the depth to the uniform theoretical completion depth was subtracted from the actual depth to the midpoint of screen for each well. This was multiplied by the vertical gradient (negative in this case) and added to the water table elevation. The resulting value for each well would therefore represent the standing water level if that well were completed to 51.5 feet BGS. As an equation: Equation 2: ((Depth to screen midpoint - 51.5) * (vertical gradient)) + water table elevation Flow Direction Comparison, March and July 2003: Water elevation contours of the uncorrected water level data gathered during the March 2003 quarterly monitoring event are shown in Figure 2. Flow direction from these contours is approximately N. 50 W. at a gradient of 0.016. The respective potentiometric elevations for MW-11S and MW-11D this quarter were 1,740.56 and 1,740.47. Therefore, the vertical gradient by Equation 1 is:

1,740.47 - 1,740.56 38.5 - 28.5

This yields a vertical gradient at MW-11 of -0.009. After correcting the elevations in Figure 2 by Equation 2, the resulting data indicated a flow direction of N. 77 W. at a steeper gradient of 0.03 (Figure 3).

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1,740.351,

740.

201,

740.

601,

741.

001,

741.

40

Flow Direction = N.77 W.Gradient = 0.03

CARDLOCK STATION

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Figure 3 – Intermediate Aquifer: Corrected Data, March 2003. An additional quarter of data was gathered prior to choosing the locations of further monitoring wells. The July 2003 uncorrected flow direction was directed more to the northeast near MW-8 and MW-9, but curved around to the northwest near MW-10 and MW-11. Overall, flow direction for the uncorrected data from July 2003 (Figure 4) was similar to that from March 2003.

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1,741.

45

1,741.70 1,741.60

1,741.73

1,740.86

1,741.13

Flow Direction = N. 35 W.

Gradient = 0.011

CARDLOCK STATION

COUNTY YARD

Figure 4 – Intermediate Aquifer: Uncorrected Data, July 2003. For July 2003, the measured elevations were 1,741.23 and 1,741.13 for MW-11S and MW-11D. Therefore, the vertical gradient this quarter was -0.01. The corrected flow direction from July 2003 (Figure 5) was virtually identical to the corrected flow direction for March 2003. Once again, the corrected data displayed a higher gradient at 0.019, while the gradient for the uncorrected data was 0.011.

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1.80

1,741.89

1,741.71

1,740.86

1,741.00

Flow Direction = N.80 W.Gradient = 0.019

CARDLOCK STATION

COUNTY YARD

Figure 5 – Intermediate Aquifer: Corrected Data, July 2003 Fuel Constituent Concentrations: The primary contaminant of concern at this site is MTBE. The primary Maximum Contaminant Level (MCL) for MTBE in California is thirteen parts per billion (ppb), while the secondary standard (taste and odor threshold) is five ppb. Analytical results for the domestic well just south of the cardlock station routinely returned concentrations in excess of the primary MCL. In addition, a domestic well southwest of the County Yard consistently contained concentrations of about one ppb. Therefore, prior to March 2003, it was assumed groundwater flow direction in the deeper aquifers was directed southwest. Fuel constituent distribution by well for March and July 2003 is summarized in Table 2 below:

Table 2 - Laboratory Analytical Results for Groundwater, March and July 2003 Well No. TPHg Benzene Toluene Ethylbenzene Xylenes MTBE TAME TBA March 2003: Intermediate Aquifer: MW-8 ND ND ND ND ND ND ND ND MW-9 ND ND ND ND ND ND ND ND MW-10S 150* ND ND ND ND 350 2.5 ND MW-11S 120* ND ND ND ND 350 ND ND MW-11D 170* ND ND ND ND 470 2.9 ND Deep Aquifer:

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MW-7 79* ND ND ND ND 190 0.96 58 MW-10I 480* ND ND ND ND 1,300 8.7 53 MW-10D 370* ND ND ND ND 980 6.9 ND MW-12D ND ND ND ND ND 7.2 ND ND

July 2003: Intermediate Aquifer: MW-8 ND ND ND ND ND ND ND ND MW-9 ND ND ND ND ND ND ND ND MW-10S 130 ND ND ND ND 370 2.3 ND MW-11S ND ND ND ND ND 0.71 ND ND MW-11D 160 ND ND ND ND 420 2.3 ND Deep Aquifer: MW-7 ND 0.78 2.5 0.69 3.8 28 ND ND MW-10I 760 ND ND ND ND 2,300 15 100 MW-10D 400 ND 5.8 ND 5.1 1,100 7.7 ND MW-12D 840 43 44 8.0 40 1,800 6.0 ND September 2003: Deep Aquifer: MW-12D ND ND 2.7 ND 1.9 24 ND 23 All results presented in parts per billion (ppb) MTBE results by EPA method 8260ND = below detectable concentrations * = gasoline value due to MTBE The July 2003 analytical results from MW-12D were startling given the much lower results prior to completion and from the previous quarter. Therefore, an additional sample was gathered the following month and returned results more in-line with the March 2003 quarter. Deep aquifer wells in this area typically yield 20 to 50 gallons per minute, while intermediate aquifer fractures produce less than five GPM. Inspection of well logs on file with the State showed the nearby domestic wells were all completed into the deep aquifer, but several could also be cross completed with the intermediate aquifer. The higher concentrations present in the deep aquifer necessitated further investigation and some form of remediation. If the actual flow direction is oriented more northwesterly as the uncorrected data indicates, MW-8 and MW-9 would be roughly upgradient from MW-10 and MW-11. However, these wells have perennially been unimpacted. This posed a minor problem. If such a flow direction were correct, either the MTBE source was not from the cardlock station, or the transport mechanism was unusually complex. Additional Monitoring Points: Since the flow direction obtained from the corrected data suggested a shorter, less complex flow path from the station and a better fit to local topology, additional well locations were proposed based on this data. These locations were approved by the State Regional Water Quality Control Board (Figure 1), and drilling commenced in December 2003. Monitoring well MW-13 was chosen as an upgradient well by the corrected flow direction, while MW-15 was the downgradient well. The large separation of the lateral MW-14 was necessitated by another intervening septic field and required county setbacks. One four-inch extraction well was planned for the intermediate aquifer, but neither attempt intersected the large producing fractures in MW-11. These borings were converted to MW-17 and MW-18. Deep aquifer extraction well EX-2 was successfully completed into a large producing fracture. Subsequent pump testing indicated EX-2 was capable of producing in excess of 30 GPM, with a radius of influence greater than 295 feet after one minute at 29.5 GPM and at least 450 feet after six hours. After logging and completing all the wells as detailed above, the next quarter of monitoring was performed during February 2004. For the intermediate aquifer, MTBE was discovered in higher concentrations in both MW-13S and MW-17D. Downgradient monitoring well MW-15 also contained concentrations of MTBE above the MCL. 631

In the deep aquifer, MW-15I and MW-15D both contained concentrations of MTBE easily above that detected laterally in MW-12 and MW-14. Analytical results for the intermediate aquifer from this monitoring event are summarized in Table 3 below:

Table 3 - Laboratory Analytical Results for Groundwater, February 2004 Well No. TPHg Benzene Toluene Ethylbenzene Xylenes MTBE TAME TBA Intermediate Aquifer: MW-8 ND ND ND ND ND ND ND ND MW-9 ND ND ND ND ND ND ND ND MW-10S ND ND ND ND ND 120 ND ND MW-11S ND ND ND ND ND 4.2 ND ND MW-11D ND ND ND ND ND 180 ND ND MW-13S 67 ND ND ND ND 290 ND ND MW-14S ND ND ND ND ND ND ND ND MW-15S ND ND ND ND ND 12 ND ND MW-17S ND ND ND ND ND 23 ND ND MW-17D 130 ND ND ND ND 300 1.3 ND MW-18S ND ND ND ND ND 13 ND ND MW-18D ND ND ND ND ND 46 ND ND Deep Aquifer: EX-2 130 ND ND ND ND 310 1.4 ND MW-7 69 ND ND ND ND 250 1.1 ND MW-10I 430 ND ND ND 0.84 1,500 14 58 MW-10D 110 1.2 1.4 1.5 4.4 360 1.7 ND MW-12D ND 0.93 1.1 1.5 4.2 29 ND ND MW-13I 220 0.74 0.87 0.83 1.9 980 ND ND MW-13D 210 0.74 0.89 1.2 3.4 850 2.6 ND MW-14D ND 0.98 1.2 1.7 4.7 9.7 ND ND MW-15I 270 12 7.2 5.1 16 220^ NA NA MW-15D 140 0.69 0.74 0.95 2.7 450 1.5 ND All results presented in parts per billion (ppb) MTBE results by EPA method 8260ND = below detectable concentrations ^ = MTBE value by EPA method 8020 F ebruary 2004 Water Table Contours: Contouring of intermediate aquifer water levels utilizing corrected data from the February 2004 quarterly monitoring is presented in Figure 6. Even with the additional wells, corrected flow direction only varied by about three degrees between the three monitoring events. Horizontal gradient is again fairly steep at 0.043. The wells in the County Yard still do not appear to be directly downgradient from the cardlock facility. However, flow direction in the more highly contaminated shallow aquifer follows the slope northeast of the site and does flow southwest under the cardlock station. This presents the possibility impacted water from the shallow aquifer is being transmitted to the intermediate aquifer southwest from the original release near MW1.

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1,765.78

1,763.651,771.30

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1,769.18

1,768.31

1,768.56

1,768.35

1,768.87

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Fence Line

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Figure 6 - Intermediate Aquifer: Corrected Data, February 2004. Conclusions:

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Prior to March 2003, only two wells existed in the intermediate aquifer. With the addition of MW-10 and MW-11, flow direction for this aquifer could be evaluated. Initial contouring indicated a northerly flow direction inconsistent with local drainages and the location of the presumed source point of the MTBE contaminant. Although three wells existed in the deep aquifer, flow direction could not be evaluated as the wells were directly in line with one another. Additional deep wells were added at the beginning of 2004, but only data from the February 2004 quarter of monitoring was available as of this writing. Flow direction in the deep aquifer appears to be west-southwest, but with no additional data for comparison, those figures are not be presented here. With vertical distance between completions not negligible in relation to horizontal separation, it was deemed likely water table maps prepared from the raw data contained components of both vertical and horizontal flow. To test this hypothesis, water level measurements in all intermediate aquifer wells were corrected to a uniform depth by mathematical application of measured vertical gradient. The resulting “corrected” data represented water level measurements at a discrete standard depth and would ideally be devoid of vertical gradient influence. The same procedure was followed in the subsequent quarter, with noticeably similar results. Subsequent wells were then planned and installed based on flow direction obtained from the “corrected” data. These wells, installed prior to February 2004, enabled evaluation of the potentiometric surface over a much larger area. Flow direction, however, remained nearly identical. Of these new intermediate aquifer wells, downgradient well MW-15S was found to contain MTBE in concentrations above the MCL. Also, upgradient wells MW-13S and MW-17D contained higher concentrations of MTBE than the other wells in this aquifer. Again, MW-8 and MW-9, although roughly upgradient by the uncorrected data from prior quarters, were both unimpacted. Judging by the presence or absence of the MTBE contaminant, this vertical gradient correction process allowed accurate prediction of the primary flow and transport direction at the investigation site. Corrected versus uncorrected data for the February 2004 quarter made very little difference in the flow direction. This may be partly due to the initial four wells being very nearly parallel to the water table contours, as well as the relatively steep horizontal gradient. Nevertheless, if future quarters of monitoring continue to return similar results, it will indicate the correction method presented here correctly identified horizontal flow direction with a limited number of fairly closely spaced wells. Acknowledgements: The authors would like to thank James Olbinski for his work in drafting the figures contained in this paper. Biographical Sketches William Earl Ackland graduated cum laude with a B.S. in geology from Oregon State University in 1993. He then earned and served a National Association of Geology Teachers Student Summer Internship with the USGS in Cheyenne, Wyoming performing groundwater remediation investigations at F.E. Warren AFB. In 2001 he earned an M.S. in geology: hydrogeology from Portland State University. Mr. Ackland is currently employed by HerSchy Environmental, Inc. in the capacity of project manager and hydrogeologist. Herman Schymiczek obtained a B.S. and M.S. (with distinction) from California State University, Northridge in 1980 and 1983, respectively. He has over 20 years of experience in natural resource evaluation and production and site assessment and remediation of contaminated soil and groundwater. He is a Registered Geologist in California and Oregon, and a Certified Hydrogeologist and Certified Engineering Geologist in California. He is currently president of HerSchy Environmental, Inc., a firm which he founded in 1995.

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HerSchy Environmental, Inc. P.O. Box 229 Bass Lake, CA 93604-0229 herschy@sti.net (559) 641-7320 phone (559) 641-7340 fax