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Sandy Nettles.pdf

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<strong>Sandy</strong> <strong>Nettles</strong>, P.G.Ocean Earth Technologies5 th Annual Caribbean Environmental Forum


Introduction to well construction techniques andaquifer concepts Concepts of a comprehensive hydrogeologicevaluation Aquifer testing and analyses Specific Capacity Testing Aquifer performance testing Factors Affecting Results Well Efficiency Partial Penetration Conclusions


Understanding of site-specific hydrogeologyis a must Withdrawal from a well will impact localgroundwater and surrounding lakes, streams,wetlands, and rivers Withdrawal can also impact ground stability,terrestrial vegetation, and water quality In coastal aquifers, withdrawal can affect thelocation of the fresh/salt water interface


Compile, review, and evaluate all existinghydrogeologic data available for the site andgroundwater basin Generate maps of geologic stratigraphy andaquifer thickness based on existing data Use topographic and geologic maps togenerate a groundwater basin map as well asan associated surface water basin map


Soil lithology and distribution Water supply well drilling logs, down holegeophysical logs of wells, monitor welllithology logs, geotechnical boring logs Aerial distribution of aquifer, depth ofaquifer, stratigraphy of aquifer


Supply well water levels Pumping rates and duration Ground water quality Pump test information, aquifer coefficients,soil permeability Monitor well construction logs, water levels


WatershedBoundaryTopography


Rainfall Temperature Humidity Barometric pressure Evapotranspiration rates


Drawdown fromadjacent pumping wellEffects of Tidal Flux


Multi-Electrode Electrical Resistivity (MER) Non-invasive geophysical technique that recordssediment type and thickness, depth to rock,groundwater levels, gross water quality MER will: Define the thickness and extent of the aquifer Delineate the distribution of sediments throughoutthe aquifer Delineate zones of increased porosity Delineate zones of possible contamination Delineate the fresh/salt water interface


MER Transect across an aquifer system in St. Kitts. MER Transect showsthree distinct geologic units as well as the fresh/salt water interface


The amount of water that can be withdrawnfrom the aquifer depends chiefly upon: The aquifer’s capability to transmit water fromareas of recharge to the points of withdrawal The amount of water available in the areas ofrecharge to replace the water that moves topoints of withdrawal The amount of water available from storage asthe water level declines


To establish the Water Crop or Safe Sustained Yield of anaquifer requires the following hydrogeologic information: Delineation of the basin’s hydrologic boundary Definition of the underlying geology and aquifer units Determination of aquifer hydrologic parameters Soil type, distribution and hydrologic characteristics Rainfall rate and distribution Topography and geomorphology Evapotranspiration rates and distribution Surface water runoff Groundwater recharge rate Groundwater withdrawal locations, rates and periodicity


Water flow in an aquifer system is governedby the permeability of material, flowgradient, thickness of the aquifer andhydraulic head or artesian pressure Important to know geologic lithology andstratigraphy of aquifer and any confiningstrata Pump tests are conducted to determineaquifer parameters


Transmissivity (T): rate at which water ofprevailing kinematic viscosity is transmittedthrough a unit width of the aquifer under a unithydraulic gradient, i.e. gpd/ft Storage (S): volume of water released fromstorage, or taken into storage, per unit of aquiferstorage area per unit change in head In unconfined aquifer, S is equal to the Specific Yield Specific Yield: ratio of one volume of waterwhich, after being saturated, it will yield bygravity to its own volume


The drawdown in a well at different times and differentdischarges (establishes well field operation program). Well efficiency. The radius of influence of the cone of depression onadjacent or future wells The effect of well drawdown on surface water levels todetermine environmental impacts of groundwaterwithdrawals. The effect of well withdrawals on the fresh/salt waterinterface (determines safe sustained yield, maximum welldrawdown, maximum pumping rates and wellfieldoperation program). The rate of contaminant migration through thegroundwater system.


Prior to starting the aquifer performancepumping test, a specific capacity test shouldbe performed Determine the optimum pumping rate for theaquifer performance test Pumping the well at one rate until the drawdownstabilizes (generally an hour or two), thenincreasing the pumping rate and holding thatpumping rate until the drawdown stabilizes


The maximum anticipated drawdown for theaquifer performance test. The pumping rate that will effectively stress theaquifer. The rate of water level decline in the pumpingwell and monitor wells at the beginning of thetest. To determine if the monitor wells are ideallylocated To determine well efficiency and design pumpdepth setting and design pumping rate.


An aquifer performance test consists ofpumping a test supply well for a specificperiod of time at a constant pumping rateand measuring water level declines in thepumping well as and an array of monitorwells. The pumping test is run until thedrawdown in the pumping well stabilizes at aconstant elevation


The most widely used analytical solution forcalculating the aquifer parameters of T, S and Lwas developed by C.V. Theis (1935). The Theisnon-equilibrium formula (Q = K(H 2 -h 2 )/1.366 logR/r), developed for artesian aquifers, has beenmodified to fit varying field conditions. For thisequation (Figure A-2 unconfined): Q = well yield or pumping rate in m 3 /day K = Hydraulic conductivity of the aquifer inm 3 /day/m 2 (m/day) H = Static head of water in m h = depth of water in the well while pumping (m)


The intake portion of the well penetrates theentire aquifer. The cone of depression has reachedequilibrium so that drawdown and radius ofinfluence of the well do not change withcontinued pumping at a given rate. The discharge is constant. The aquifer receives no recharge during thecourse of the aquifer performance test.


Pre-pumping water levels must be taken at leasthourly (tidal flux correction) Maintain a constant pumping rate Record water levels in the pumped well and monitorwells to an accuracy of 0.003 meters Record the water levels at the proper time increments(logarithmic scale) Compare recovery data with the drawdown data. Pump the well for a minimum of 72 hours for a watertable aquifer Discharge must be diverted away from test area toprevent artificial recharge of aquifer


The amount of drawdown in a well is areflection of the transmissivity and storagecharacteristics of the aquifer as well as thenature of the well construction Friction loss in formation will increasedrawdown Friction loss also occurs within gravel pack andscreen Proper well construction is important!


Poor well development procedures that donot successfully remove the drilling mud andformation fines from the geologic formation. Well screens that do not fully penetrate theproducing zone. Well screens that are not gravel packed. Well screens with limited open area (slotopening area per foot of pipe).


Longer screen lengths, larger slot openingsand gravel packing the screens Perform extensive development The use of a rounded gravel that issignificantly larger in diameter than thescreen slot Outer casing installed to the depth of the topof the well screen


Recommended OpenHole Well ConstructionCurrent WellConstruction


Initial collection of hydrogeologic data isimperative to wellfield design Geophysical mapping provides a rapid andnon-invasive method to map geology Aquifer performance testing provides anessential understanding of the aquifer Well construction specifications cansignificantly impact well efficiency Well location is as important as all otherfactors, especially in coastal aquifers

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