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HomeMy WebLinkAboutSOUTHPOINTE S-10950-2Southpointe 5-10950-2 AQUIFER TEST AND DATA EVALUATION FOR THE PROPOSED VILLAGES WEST SUBDIVISION COMPLETED FOR POTTER CREEK LAND COMPANY 16420 St. James Place ANCHORAGE, AK 99516 907-345-1842 April 29, 2004 INTRODUCTION An aquifer test was conducted on April 7 through April 10, 2004, on property owned by the Potter Valley Land Company and proposed for development into Villages View Estates Subdivision. This test was completed as a supplement to information gathered on September 5 through September 7, 2003. The second test was performed to provide additional data and to improve the quality of the aquifer test. The property is proposed for development into 21 single-family home lots. The purpose of the aquifer test was to evaluate the long-term capacity and nature of the aquifer and to determine the capacity of wells to be constructed to serve the individual lots in the subdivision and impacts to wells and property in the surrounding area. The scope of the test included a short-term aquifer test to estimate an appropriate discharge rate for the long-term test. Once the discharge rate was determined a long-term test was to be conducted to evaluate the production capabilities of wells in the proposed subdivision and impacts to surrounding wells in the area. The long-term test was conducted over a period of 72 hours and included the pumping and monitoring of the production well and the monitoring of drawdown in both the production well and two outlying observation wells. In addition, a well located approximately 1,000' to the west of the production well in Villages View Estates West Subdivision was also monitored for drawdown impact during the aquifer test. The proposed Villages West Subdivision is located immediately east of The Villages Scenic Parkway and south of Potter Valley Road. Much of the surrounding property in the area is served by the Municipal water system. Three lots to the west currently have wells on the property. Terrasat, Inc conducted a study and aquifer test on those wells in late 2001. A well included in this study was monitored during our aquifer test for impacts to drawdown during our aquifer test. We have reviewed the report completed by Terrasat and relied on some of the assumptions and conclusions made during their study in our analysis. The terrain of the Villages West Subdivision is very similar and mostly identical to that studied during the Terrasat, Inc. analysis. ACKNOWLEDGEMENT All formulas utilized in this report unless otherwise noted are taken from the book entitled "Groundwater and Wells" written by Fletcher G. Driscoll, Ph.D. The book was copyrighted in 1986 and the referenced issue is the sixth edition completed in 1995. METHODOLOGY M -W Drilling, Inc. completed three wells in the proposed subdivision. These wells were located on Lot 1, Block 1, Lot 1, Block 2 and Lot 2, Block 2. The locations for the wells were pre -approved by Mr. Jim Cross, P.E. of the Municipality of Anchorage Development Services Department On -Site Water and Wastewater Program. The location map is included as an addendum to this report, which details the location and separation distances between the wells. Each well is constructed of 6" steel casing which is grouted into bedrock. The wells are completed open hole in bedrock and intercept water from fractures in the bedrock. Villages West Subdivision Aquifer Test Page 1 of 6 The well on Lot 1, Block 1 is 305' deep with 6" casing extended to nearly 21' below the surface. The static water level is 40.55' below the top of casing. This well is located approximately 150' west of the well on Lot 2, Block 2. The ground elevation at the wellhead is approximately 1040'. The well on Lot 2, Block 2 is 342' deep with 6" casing extended to 20.35' below the surface. The static water level is 47.12' below the top of casing. This well is located approximately 300' south of the well on Lot 1, Block 1. The ground elevation at the wellhead is approximately 1058'. The well on Lot 1, Block 2 is 206' deep with 6" casing extended to 20.2' below the ground surface. The static water level is 5' below the top of casing. This well is located 300' north of the well on Lot 2, Block 2 and approximately 330' northeast of the well on Lot 1, Block 1. The ground elevation at the wellhead is approximately 1,035'. AQUIFER TEST An aquifer draw down and recovery test was conducted on the well on Lot 2, Block 2 from April 7 to April 10, 2004. The well was pumped at a continuous rate of 3.0 gallons per minute for 72 hours. The two outlying wells were monitored during the test for draw down. The total draw down in the production well (Lot 2, Block 2) at the end of 72 hours was 108.24' to a depth of 156.65'. The total depth of the well is 342'. Full recovery was noted in the well less than 6 hours later. The drawdown in the well on Lot 1, Block 1 was monitored during the test and was measured at 1.03'. At the end of the of the 72 hour test, however, the static water level in the monitor well had recovered completely and surpassed the level recorded at the beginning of the test. This can probably be attributed to recharge of the aquifer during the test. No drawdown was noted in the well on Lot 1, Block 2. In addition, no drawdown was measured in the well located to the west approximately 1,000' away in Villages View Estates West Subdivision. Data recorded from the production well (Lot 2, Block 2) during the test yielded a calculated transmissivity of 12.77 gallons per day per foot. The well on Lot 1, Block 1 was pumped for a period of 24 hours between September 9 and 10, 2003. The pumping rate was set at 3.5 gallons per minute and maintained over the duration of the test. Again, the two outlying wells were monitored for draw down during the test. The total draw down in the production well (Lot 1, Block 1) was measured at 83.76'. A total drawdown of 1.7' was measured in the well on Lot 2, Block 2 and no appreciable draw down was measured in the well on Lot 1, Block 2. The calculated transmissivity for this well was noted at 20.09 gallons per day per foot. The well on Lot 1, Block 2, was pumped for a period of 24 hours between September 9 and 10, 2003. The pumping rate was initially set at 12 gallons per minute but was gradually scaled down to 3.0 gallons per minute for the last 17 hours of the test. Again, the two outlying wells were monitored for drawdown during the test. The total drawdown in the production well (Lot 1, Block 2) was measured at 149.67'. No appreciable drawdown was measured in either of the outlying wells during the test. The static water level in the well recovered to near its pretest level after 17 hours. Drawdown data obtained from the test was erratic due to changes in the pumping rate and inflows into the well resulting in dramatic changes to the static water level. The Villages West Subdivision Aquifer Test Page 2 of 6 estimated calculated transmissivity for the well was noted at 12.75 gallons per day per foot with an estimated storativity factor of .000001. DATA INTERPRETATION Transmissivity (T) of an aquifer is defined as the rate at which water flows through a vertical strip of the aquifer V wide and extending through the full saturated thickness, under a hydraulic gradient of 1 (100%). During the aquifer test on the production well on Lot 2, Block 2 it was noted the draw down in the well on Lot 1, Block 1 was measured at slightly more than 1.0'. As previously mentioned, however, the static water level in the monitor well completely recovered and exceeded the level at the beginning of the test prior to the completion of the aquifer test. We completed our calculations based on the drawdown observed in the well prior to recovery, but before the end of the aquifer test. The coefficient of transmissivity is calculated from the pumping rate and the slope of the time -draw down graph in the observation well. The coefficient of transmissivity for our aquifer test was calculated at 416.8 gallons per day per foot. This value seems high and is affected by the minor amount of drawdown noted in the observation well during the aquifer test. The static water level dropped only 1' after 18 hours of pumping. It then held constant for an additional 10 hours while pumping continued in the production well. The static water level then gradually rose up to and past the original level while pumping continued until the test was completed after 72 hours. The final static water level was measured at 40.46 at the completion of pumping compared to 41.0 at the beginning of the test. It is apparent from the readings the monitor well was affected by recharge effects or other outside influences not related to the production well. We have therefore chosen not to rely on this information in determining long-term impact on wells in the surrounding area. We have chosen to utilize data obtained in the production well to determine transmissivity of the aquifer. Transmissivity was determined utilizing the production well on Lot 2, Block 2 by multiplying the average production rate of 3.0 gallons (Q) per minute by 264 and dividing by the slope of the time -drawdown graph expressed as the change in drawdown between any two times on the log scale whose ratio is 10. In this case the total drawdown over the test was 62' (AS). This value was used to determine the transmissivity between the production well and the observation well. T = 264Q AS T = 264 (3.0 GPM) 62' T = 12.77 Gallons/Day/Foot This transmissivity value compares favorably with that obtained in the Terrasat Report completed on the Villages View Estates West Subdivision immediately to the west. Transmissivity values noted in that report range from 3 to 18 gallons per day per foot. Villages West Subdivision Aquifer Test Page 3 of 6 Storativity represents the volume of water released from storage, or taken into storage, per unit of aquifer storage area per unit change in head. Calculated storativity values from observation well data give the best approximation for the area between production and observation well. The storativity value using data from Lot 1, Block 1 during the pumping of Lot 2, Block 2 was calculated at .000945. The coefficient of storativity was calculated from the time -drawdown graph using the zero drawdown intercept of the straight line of the slope. The intercept using our data was determined to be 250 minutes or .17 days (t ). This number was then multiplied by the transmissivity determined above of 419.2 gallons per day per foot (T) and the constant .3. This total was then divided by the total of the distance between the production and observation well squared (R). The coefficient of storativity for our aquifer test was then determined to be .000945. Again the storativity determined from the monitor well on Lot 1, Block 1 is influenced by the Transmissivity value attained and is probably not accurate due to the slight impact the production well appears to have had on the monitor well. We again used the information from the recovery of the production well on Lot 2, Block 2 to determine storativity. S = .3Tt R S = .3(10.42)(.004) (150) S = .000001 The storativity determined from the production well on Lot 2, Block 2 was found to be .000001 based on the recovery of the well verses time after the aquifer test had been completed. This figure compares favorably with the storativity values achieved from production data completed on the wells on Lot 1, Block 2 and Lot 1, Block 1. Storativity values achieved in the Terrasat Report for Villages View Estates West Subdivision ranged from .000037 to .0016. In that study, however, the radius used to determine storativity was 80' as opposed to our radius of 150'. In addition, the formula utilized in our study varied from that used by Terrasat. The formula for storativity for our report is found in "Groundwater and Well" by Fletcher G. Driscoll, Ph.D. in Chapter 9, Well Hydraulics. Based on the transmissivity and storativity values calculated from data collected during the aquifer test we are confident that each well should be able to support a minimum pumping rate of 1.0 gallon per minute for 10 years of continuous pumping. SPECIFIC CAPACITY AND LONG TERM YIELDS The effects of long term pumping for a single family home are calculated using a typical domestic use rate of 450 gallons per day (Q), the value of T obtained from the data obtained from the production well on Lot 2, Block 2 (10.42 gallons per day per foot) as an estimate of the aquifer capability and a storativity value of S=.000001. After 30 years of pumping negligible draw down is calculated a distance of 100' away from the pumping well. This assumes no recharge, which is very conservative. The expected Villages West Subdivision Aquifer Test Page 4 of 6 impacts from any single domestic well are therefore expected to be extremely minor. Each well should be able to support a pumping rate capable of producing 450 gallons per day or .3125 Gallons per Minute. IMPACT PROJECTIONS The theoretical specific capacity of a well discharging at a constant rate in a homogeneous, isotropic, nonleaky artesian aquifer infinite in areal extent is given by the equation below. This equation is taken from 'Practical Aspects of Groundwater Modeling" Second Edition, Copyright 1985 by William C. Walton a consultant in water resources. The equation assumes the production well penetrates and is uncased the total saturated thickness of the aquifer. It also assumes the well loss is negligible and the effective radius of the production well has not been affected by the drilling and development of the production well and is equal to the nominal radius of the production well. CALCULATION OF SPECIFIC CAPACITY FOR A PUMPING WELL Q/s = Specific Capacity (Gallons per Minute per Foot) Q = Discharge (Gallons per Minute) s = Drawdown (Feet) T = Coefficient of Transmissivity S = Coefficient of Storativity r = Nominal Radius of the Well (Feet) t = Time after Pumping Started (Minutes) No 264 log ( Tt ) —66.1 2,693 r S 1 Hour = .008 GPM/Foot 6 Months = .0046 GPM/Foot 1 Year = .0045 GPM/Foot 10 Years = .0040 GPM/Foot 20 Years = .0039 GPM/Foot CONCLUSIONS The groundwater conditions at the proposed subdivision have been studied and tested on site with three newly constructed wells. These wells have been found to be capable of yielding far in excess of the minimum required supply for a single family home as mandated by the Municipality. Existing aquifer testing, water level, well yield and water budget data all indicate that sufficient water supplies are available from wells to be constructed in The Villages West Subdivision without affecting neighboring wells. All new wells are expected to exceed Municipal requirements for single-family homes. The three wells completed on this property were found to have production rates of 3.0, 3.4 and 3.5 gallons per minute. These rates far exceed the Municipal requirement for Villages West Subdivision Aquifer Test Page 5 of 6 single-family homes. In addition, these rates are much greater than the average well production rates for wells in the Anchorage area. Data collected during the aquifer test indicates that little or no impact was seen in the observation wells during the 72 hours of continuous pumping of the production well. A third well located approximately 1,000' to the east was similarly monitored during the test with no fluctuation noted during the test period. This data would suggest that the wells operating independently would have little affect on each other. Recovery of the production well after 72 hours of constant pumping required only 4 to 6 hours indicating recharge of the aquifer is very good and a substantial amount of water is available to support this subdivision. Conclusions drawn from the aquifer test completed in the Villages View Estates West Subdivision stated long term pumping rates of 1.0 gallon per minute should not cause well interference beyond available head. Our aquifer test showed similar results to the Terrasat test, but with much less drawdown in the monitor well. We therefore conclude similarly, that a 1.0 gallon per minute production rate per well will have no impact to adjacent wells. This rate is adequate for a home with up to nine bedrooms according to Municipal standards. A recent study completed on the subdivision immediately to the west of this proposed subdivision concluded that a connection between rock aquifer systems in the area is very unlikely. This is supported by the apparent north -south trend in local fracture systems as interpreted from aerial photographs. This indicates that the hydraulic connection in an east -west direction is unlikely. No data, however, exists to conclusively state the water wells in this subdivision are not hydraulically connected to adjacent water wells. Data obtained during the aquifer testing and presented herein are representative of conditions at the time of testing and should be considered representative only of that particular period in time. Changes will occur to the property over time, which will affect subsurface conditions and may impact conclusions drawn in this report. Michael E. Anderson, P.E. Villages West Subdivision Aquifer Test Page 6 of 6