EXPLORATION AND PUMPING TEST RESULTS FOR A HIGH-YIELD CRYSTALLINE BEDROCK WELL IN TOBAGO

Transcription

1 EXPLORATION AND PUMPING TEST RESULTS FOR A HIGH-YIELD CRYSTALLINE BEDROCK WELL IN TOBAGO Randolph Sankar and Learie Jadoo Water and Sewer Authority of Trinidad and Tobago Joseph Ingari, Roland B. Hoag, Claude Cormier and Robert A. Bisson EarthWater Technology International, Inc. / HydroSource Associates, Inc. Abstract Trinidad's water supply system currently (October 2000) provides consumers with 183 million imperial gallons per day (imgd) of potable water, of which 48 imgd, or 26%, is derived from approximately two hundred wells drilled into alluvial sands and gravels, sandstones and limestones. Prior to year 2000, Tobago's water wells contributed only 2% of the island's supply and were universally located in alluvial aquifers. In 2000, newly drilled bedrock wells increased groundwater's contribution to more than 20%. This was accomplished in nine months by WASA, using a novel contracting method [1] that encouraged the application of innovative technologies to new water discovery [2]. INTRODUCTION Historical contributions to the Trinidad water supply system from groundwater sources developed using conventional exploration and drilling technologies are shown in Table 1. The table shows a modest average yield for production wells (less than imgd per well overall) and a very high proportion (99%) of water produced from sand and gravel quifers over bedrock aquifers.

2 Groundwater development efforts in Tobago prior to 2000 were even less successful than in Trinidad, with water wells contributing only 2% of Tobago's total water supply. In Tobago, most known alluvial aquifers are modest in size and confined to narrow coastal margins and stream valleys, primarily in the northern and eastern portion of the island. Previous groundwater assessments of Tobago based on traditional hydrological concepts and using conventional groundwater mapping techniques determined that no further significant groundwater resources were available on Tobago. The Government Farms Favorable Zone lies within a Megawatershed comprising Cretaceous Age submarine volcanic assemblages and secondary metasedimentary basement rock. Local volcanic and metasedimentary foliation/bedding generally trend northeasterly and dip at moderate angles to the southeast. The Pliocene Age Rockly Bay Formation, comprising interbedded, massive clays and minor shell and gravel horizons, forms a confining cover over the

3 volcanic bedrock. Rockly bay formation thickness varies from feet or more in the study area. The bedrock in the area has been subjected to strong brittle deformations associated with the active Southern Tobago fault system. Local volcanic and metasedimentary formations (and possibly the Rockly Bay formation) are cut by a northwest striking/northeast dipping normal fault identified in outcrops north of Government Farms. A bedrock test well (GF #6), was sited to intersect this fault at depth. Megawatershed Recharge Recharge within the megawatershed was determined by compiling and correlating rainfall, runoff and evapotranspiration data for the entire island [2]. The minimum annual recharge potential from the megawatershed supplying the Government Farms site was assessed at 0.6 imgd Geologic and Geophysical Field Work Quantitative geologic and brittle feature mapping investigations were carried out in the Government Farms favourable zone and within the Government Farms megawatershed area. A variety of previously and newly mapped faults and fractures were identified including a northwest striking normal fault and shallow dipping northeast trending fractures parallel to volcanic layering planes. Several complimentary geophysical surveys were undertaken along multiple traverses of the Government Farms area to confirm geologic mapping results and identify potential drilling sites. Specialised interpretations of magnetic, gradiometric, VLF electromagnetic, microgravity and earth resistivity surveys were employed in conjunction with resistivity microgravity, magnetic and gradiometric results to assess overburden type and thickness, detect undulations in the bedrock surface caused by the erosion of differentially weathered volcanic units and determine the location, orientation and dip direction of fault zones. Electromagnetic survey data was used to delineate conductive horizons in bedrock and overburden and map cultural features to aid in the interpretation of resistivity surveys. The data from these surveys was processed and compared with geologic and remote sensing information to arrive at a three dimensional conceptual understanding of the study area (Figure 3). The resulting conceptual geological model confirmed the existence of the northwest trending fault, identified deep alluvial clays and established the locations of northeast striking, foliation controlled bedrock troughs. Drilling targets were selected to intersect northwest striking and easterly dipping normal faults, and the northeast striking and southeasterly dipping, foliationaligned fractures.

4 In November 1999 the Water and Sewerage Authority of Trinidad and Tobago (WASA) initiated a "success based" groundwater exploration and development with a Trinidad-USA private sector joint venture to carry out a groundwater resources assessment and then produce a minimum sustainable supply of two million imgd of high-quality, previously untapped groundwater. An experienced team of groundwater explorers applied novel hydrogeological concepts, proprietary technologies and interpretive techniques to analyse selected remote sensing, geological, hydrological, climatological and geophysical data in order to characterize the groundwater potential of Tobago. A comprehensive report describing the hydrogeology, groundwater availability and recharge characteristics of Tobago was presented to WASA [3]. The report identified 16 extensive groundwater recharge catchments, called "Megawatersheds" [4], and 16 sub-areas within these megawatersheds were categorized as most economically favourable for near term groundwater development. The megawatersheds are comprised of geometrically complex, laterally extensive arrays of interconnected fracture conduits that may underlie one or morel traditional, topographically controlled, watershed catchments. Based on exploration programme results, several wells were sited and drilled using this novel approach, yielding a combined 2.3 imgd of high quality water from volcanic bedrock aquifers in Tobago's megawatersheds. This paper summarizes the exploration, drilling and testing of one production wellfield, located on the Government Farms property (Figure 1). GOVERNMENT FARMS WELLFIELD Introduction and History The Government Farms wellfield is located on the Southeast side of Tobago, near the capital city of Scarborough and proximal to the area of greatest water need (Figure 2). The only producing public water supply wells in the southern part of the island, prior to the WASA groundwater exploration initiative, were alluvial wells drilled at Government Farms that produced a total of imgd, representing about 2% of the potable water requirement for this part of the island. At the Government Farms site, after geologic and geophysical mapping studies were completed, two wells were constructed: an eight (8) inch diameter test/production well and an eight (8) inch diameter monitoring well. The production well underwent "step" and "constant rate" pumping tests. Water quality was assessed during pumping and at the completion of the 72-hour constant rate [at Q = 420 imperial gallons per minute (igpm)] pumping test. A natural spring, located adjacent to pre-existing gravel well "GF#3", flows at an estimated average rate of igpm. In the past, little effort was made to measure the flow or to understand the underlying geologic source of the spring. The locations of the two original alluvial wells, spring, new bedrock well and bedrock observation well shown in Figure 2.

5 WELL DRILLING Well construction was undertaken at the primary target site using a specialized drilling rig enabling simultaneous drilling and emplacement of conductor casing through overburden deposits and into competent volcanic bedrock. Drilling to target depths was completed with a rotary percussion hammer or hardened roller bits. One test/production well and one monitoring well were constructed to intersect a northwest trending fault zone at depth. The test well intersected 77 feet of Pliocene clays and minor gravels before reaching volcanic bedrock. The well was then drilled to a final depth of 358 feet intersecting several steeply dipping fractured intervals. An airlift water yield test exceeded 850 igpm. A second well was targeted to intersect the same northwest trending fracture zone at a point midway between the fractured bedrock test well and the sea. GF #6A was drilled to a depth of 398 feet. Large water-bearing fracture zones were encountered at depth. The final airlift test yield exceeded 600 igpm. Summaries of the drilling results for GF#6 and the monitoring well GF#6A are found below. An oriented downhole video camera survey was performed at both wells and revealed the predicted steeply dipping, northwest striking and shallow dipping, foliation-aligned fractures.

6 PUMPING TESTS AND SAFE YIELDS ANALYSES Classic Well Testing Well water quality testing is aimed at establishing the suitability of produced water for the public water supply. Assessments of the level of treatment required, the acceptability from an aesthetic standpoint, the adherence to World Health Organization (WHO) health guidelines and changes in quality due to contamination are the aims of water quality testing. In the local groundwater environment, the quality parameters of greatest interest are chloride, hardness, sulphide, iron, nitrate and bacteria. Water quantity testing is conducted upon completion of a well. In isotropic porous media pumping tests are conducted by the step-drawdown method to determine well losses and by the constant rate test to determine aquifer performance. The step-drawdown test is a semi quantitative method for the analysis of well performance under turbulent flow conditions [5]. Drawdown in the well is observed at increasing discharge rates (Q) done in discrete steps. The drawdown by the Jacob equation: sw = BQ + CQ2 Where BQ = aquifer loss and CQ2 = well loss. The equation is often rearranged in the form: sw/q = B + CQ allows for a linear plot between sw/q versus Q from which the slope yields the well loss coefficient C and the constant yields the aquifer coefficient loss. Knowledge of the well loss component is very important in the design and interpretation of constant rate tests which yield information on aquifer performance. These data are also important in the design of the final well with respect to aquifer deliverability and well reliability over time. From a practical standpoint the step-drawdown test allows the optimum pumping rate of the well and the pump setting depths to be determined. After an optimum pumping rate for the well is determined from the step-drawdown test, the aquifer performance is traditionally evaluated by pumping the well at a constant discharge rate for 3-5 days while monitoring the pumping water levels in the well. The drawdown in the well under non-equilibrium conditions is governed by the Jacob modified non-equilibrium equation [5] that describes the drawdown with time at a constant discharge rate: sw = 264 Q / T where T = transmissivity in gallons per day per foot. When under these conditions, a plot of drawdown (sw) versus time on semi-logarithmic paper will yield a straight line of slope equal to 264 Q/T from which the transmissivity can be estimated. These plots of drawdown data from a

7 pumped well will be linear on semilog paper, but if pumped for a sufficiently long period of time will show the impacts of encountering natural recharge or impervious boundaries within the aquifer. Semi-log plots of 3 or 5 day constant rate test data are used to set the long term pumping rate of the well under drought conditions In some cases, however, drawdown data from pumping tests are difficult to analyse and there may be a high degree of uncertainty by regarding the actual values of Transmissivity, hence the real hydraulic potential of the aquifer. The extreme heterogeneity of many geologic formations leads to data that seem to defy rational explanation. Combining geologic information with a thorough understanding of well hydraulic theory is essential when analysing drawdown data for T. The Government Farms Pumping Test Aquifer testing and water quality measurements were performed on the 8-inch bedrock production well. Testing consisted of the following: A 100 minute interval, 600 minute step test with recovery; A 72-hour constant rate test; Constant rate test recovery monitoring; and Daily general analyses and final water quality tests The step test was performed on June 7, The well was initially pumped at a rate of 225 gallons per minute for a period of 100 minutes, the pumping rate was then increased by 75 gallons per minute for each succeeding 100 minute step. At the end of the final 600-gpm step (600 minutes of pumping) water levels had drawn down from a starting level of approximately 6.46 feet below top of well casing to feet representing a total drawdown of 15.9 feet. Discharge from the step and constant rate tests was directed into a small concrete drainage ditch and carried several hundred feet southwest and down gradient from the well. A constant rate pumping test at 420 igpm began on June 8, 2000 and continued for a period of 72 hours. During the first 30 seconds of the test, the drawdown reached feet. Total drawdown during the pumping period reached feet (Figure 4.) Following shutdown of the 72-hour constant rate test, water levels recovered from feet to 4.69 feet below ground surface in 30 seconds, representing 93 percent recovery. After 1.5 hours the well had recovered to 4.09 feet (97.5 percent of total recovery). Full recovery occurred after a period of only hours following shutdown. The total volume of water pumped during the 72 hour constant rate pumping test was 2,376,000 imperial gallons. During the pumping of the Government Farms production well #6 (GF#6) water levels in observation well GF#6a (574 feet to the east) and WASA production wells GF#3 and GF#5, and the spring were monitored. The decline of the water level in the production well during the pumping period was linear with respect to the log of time with a starting water level of 6.46 feet

8 below top of casing to an ending water level of about 13.9 feet below top of casing. GF#3 and GF#5 had been in production for many months and were shut off prior to the aquifer test. Consequently the water levels were rising (recovering) during the pumping test. Water levels in GF#3 rose from 4.96 to 4.81feet below top of casing and GF#5 rose from to feet below top of casing indicating partial isolation of the alluvial aquifer from the bedrock aquifer supplying water to the fractured bedrock production well. Water levels in the spring pond fell 0.21 feet during the test and was linear with respect to the log of time. Aquifer Parameters and Safe Yield The safe yield value for the Government Farms well was determined after prudent assessment of step test, constant rate test and recovery data characteristics and long term aquifer recharge calculations. A graph of sw/q vs Q (Figure 4 ) reveals that the Government Farms well has very low well and aquifer loss coefficients. (B =.008, C = 4.1 x 10-5). This indicates a highly transmissive aquifer and a very efficient well. Modified nonequilibrium calculations of aquifer "apparent transmissivity" values are in excess of 158,000 igpd/ft. A transmissivity this high is unheard of except in solutioned limestone systems. The range of transmissivities in alluvial aquifer systems ranges from less than 1000 igpd/ft up to 100,000 igpd/ft. Groundwater flow in fractured bedrock systems is more analogous to pipe flow rather than porous media flow. Therefore very high "apparent transmissivities" are common. These calculated transmissivities do not reflect the transmissivity of the bedrock, but that of individual fracture systems.

9 Consequently, wells intersecting individual but interconnected fractures may exhibit a large range of "apparent transmissivities". With regard to the Government Farms site, the high transmissivity coupled with the very high flow of the production well, observation well and the spring indicates that this fractured bedrock aquifer is extensive and receives its recharge through a series of interconnected faults and fractures crossing drainage divides and effectively obtaining most of its recharge from the entire megawatershed. The specific capacity (inverse of sw/q, discharge/drawdown) of the production well is also very high (26.4 igpm/ft) indicating that the well could be pumped at a much higher rate. However, much of this water may be is derived from storage within the bedrock fractures, and long term pumping at very high rates could result in a lowering of the water table, increases in drawdown and an increase in the potential for inducing saltwater intrusion. Consequently, until the well has been pumping for a period of a year or more, we maintain that long term pumping rates be no greater than the estimated megawatershed groundwater recharge amount of 0.6 imgd (420 igpm). Should water levels in the aquifer remain unchanged for a period of one year (allowing for wet and dry season adjustments in the water table configuration) an increase in the current pumping rate of 420 igpm could be warranted. The magnitude of the increase will be determined by careful analysis of long term pumping potentiometic levels in the monitoring wells and springs. WATER QUALITY Water quality samples were collected on a daily schedule during the pumping test as specified by WASA. The samples were collected by our supervising personnel, and delivered to the laboratory immediately after sampling. Analyses of samples collected both during and at the end of the test pumping for the Government farms production well showed stable water chemistry and generally excellent water quality. The results of analyses for dissolved metals and other inorganic parameters are excellent with only moderate levels of hardness and alkalinity. All chemical parameters fall well below World Health Organization guidelines and WASA potable drinking water standards. Three Month Pumping results at Government Farms Wellfield The Government Farms bedrock production well has been continuously pumped at rates higher than 0.6 imgd and monitored for three months. Continued water level monitoring of springs and observation wells reveals that the Government Farms aquifer has stabilized and that water quality has remained unchanged. On going monitoring of the Government Farms observation wells and springs indicates that aquifer water levels have stabilised at a potentiometric level of approximately 2-3 feet lower than the pre-existing potentiometric level (Figure 5). Potentiometric levels in the aquifer remain 30 feet above mean sea level. WASA continues to

10 monitor aquifer performance and expects to make a final determination of long-term pumping rate by August CONCLUSIONS A nine-month, integrated groundwater exploration program identified more than 40 imgd of new fresh water in megawatersheds underlying Tobago and successfully developed more than 2.0 imgd of drinking water near the island's principal consumer centres. High-yield bedrock production wells were drilled into fractured volcanic and metamorphic bedrock at four locations on the island. All four production facilities are fully constructed, and three are already in service at the time of this publication (October 2000). The Government Farms production well is continuously supplying water at the rate of over imgd, and its water quality has remained high quality and unchanged during a three-month pumping period after installation. SELECTED BIBLIOGRAPHY 1. Maharaj, U., Risk Management in Groundwater Development - The Tobago Example, Proceedings of the Caribbean Water and Wastewater Association Annual conference, Port of Spain, Trinidad and Tobago October 2-6, 2000

11 2. Maharaj, U., Sankar,R, Hoag, R., Bisson,R. and Ingari,J., - The Megawatershed Concept and Recharge Analyses- Sustainable Groundwater Extraction From Bedrock Aquifers in Tobago. 3. Part A. Tobago Groundwater Assessment and Groundwater Development Program, ETI report to Water and Sewerage Authority of Trinidad and Tobago Bisson, R., Space-Age Integrated Exploration and Treatment of Renewable Regional Sources of Pristine Groundwater in Fractured Rock "Megawatersheds" Desalination, Driscoll,.G., Groundwater and Wells, second Edition; Johnson Division, St. Paul, Minnesota