letter report: fractured rock aquifer pilot study · in hard rock is small, resulting in yields...
TRANSCRIPT
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LetterReport:
FracturedRockAquiferPilotStudy
2010
Preparedfor:
JonathanLearMontereyPeninsulaWaterManagementDistrict
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PurposeandScope
TheMontereyPeninsulaWaterManagementDistrict(MPWMD)requestedassistanceinordertocompleteapilotstudyinvestigatingthereliabilityandsustainabilityoffracturedrockwellswithintheCarmelWoodsandAguajitoregionsoftheDistrict.Ingeneral,thevolumeofwaterstoredwithinfracturedrockaquifersnearthegroundsurfaceistypicallyestimatedtototallessthan2%oftherockvolume,withthispercentagedecreasingwithdepthasfracturesbecomenarrowerandfartherapart.Thetotalamountofwaterinstorageinthefracturedmatrixsurroundingawellcompletedinhardrockissmall,resultinginyieldsthatmaydecreasedramatically(orevenstop)followingprolongedproductionorinstallationofnearbywellstappingintothesamewater‐bearingfractures.
Thereliabilityandsustainabilityoffracturedrockaquifersisprimarilycontrolledbythenumberandspacingoffractures(density),aperture(openingsize),thedegreeifinterconnection,orientation,thicknessandtypeofsoilcover,andthenatureanddegreeofrecharge.Themajorityofthesephysicalparametersaredifficulttodirectlyquantifyinthefield.Infact,desirablecharacteristicsalonewillnotguaranteeaquiferreliabilityandsustainability–temporalandspatialanalysisofwellscompletedinfracturedrockiscriticaltounderstandingaquiferdynamics.
AnalysisPerformed
DigitalandpaperdatasetsofwelllogswithintheCarmelWoodsandAguajitoregionsoftheDistrictwereassembledusingrecordsobtainedfromMPWMD,theDepartmentofWaterResources(DWR),andtheU.S.GeologicalService(USGS).
AcomprehensiveArcGISdatabasewascreatedandpopulatedwithrelevantdataincluding:
‐ Welllocations
‐ Lithology(rocktypeandverticaldistribution,includingthelocationofwater‐bearingfractures).
‐ Constructiondetails(includingwellborediameterandscreendepths).
‐ Pumptestdata(whenavailable)followingwellcompletion.
‐ Staticwaterleveldatafollowingwellcompletion.
‐ WaterChemistry
‐ Productionvolumesformeteredwells
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Ascoringscheme(DrawdownRatio)wasusedtonormalizepotentialproductiondatainordertoaccountfordifferencesinboreholediameters,staticwaterlevels,dynamicwaterlevelsduetopumping,andscreenlocations.TheDrawdownRatiowasusedaspartofageostatisticalanalysistoidentifyareaswherelowscoringwellsexist,thusindicatinggeographicalregionswherewaterproductionfromfracturedrocknetworkswaspoorandregionsthatwerenot.Recommendationsweremadethatwouldhelptocharacterizethereliabilityofwellsperforatedinfracturedrock.
Results
CurrentDistrictwelllogfileswerecomparedwithDWRrecordsprovidedtotheDistrictandwelllogsnotcommontobothdatasetswereidentified.AMicrosoftAccessdatabasewaspopulatedwithavailablelithology,wellconstructionandcompletiondetails,DWRwellnumber,TIFF(digitallog)number,estimatedcapacity,waterlevel,andlocationdataforwelllogslocatedintheCarmelWoodsandAguajitoregionsoftheDistrict.
Intotal,68wellswereidentifiedwithinthepilotstudyboundaries.Ofthese,10wellshadinsituhydrologicaltestingperformedonthemfollowingwellcompletioninordertoestimatehostrockphysicalparameters(namelyhydraulicconductivity)andassessthepotentialinterferingeffectsonneighboringwellswithina1,000footradius.Yearlyproductionvolumeswererecordedfor38ofthewellsspanningthetimeperiodof1992–2008.
Historicalproductiondata(Figure1)showasystematicincreaseingroundwaterextractionwithinthepilotstudyregionfrom6wellsproducing5acre‐feet/yearin2000to35wellsproducingroughly30acre‐feet/yearin2008.Thistrend,inconjunctionwithanincreasednumberofapplicationstoinstallnewwellswithinthepilotstudyboundaries,suggeststhatcurrentproductionvolumesareinexcessof30acre‐feet/year.
Lithologicalresultsshowthattherearetwoprimaryfracturedrockaquiferswithinthepilotstudyregion:shaleaquifersinCarmelWoodsandgraniticaquifersintheAguajitoregion.MergingoflithologicalstructurewithdrawdownratiosandhistoricalproductiondatashowthatwellsscreenedwithintheMontereyShalegenerallyhavesignificantlyhigherdrawdownratiosthanthosescreenedinfracturedgranite,indicatingahigherefficiencyingroundwaterextraction(Figure2).
Conclusions
ThispilotstudyhasdemonstratedthatsufficientdataexistwithintheDistrictboundariestocompilearichandcomprehensivehydrogeologicaldatabase.Thisdatabasewillbeextremelyusefulinassessingthedynamicnatureofgroundwater
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productionandrecharge,andprovidescriticalinsightintothephysicalstructureoflocalaquifers.
However,theavailabledatadonotprovidethetemporalextentnecessarytodeterminethesustainabilityoffracturedrockaquiferswithinthepilotstudyregion.Additionalwatertableelevationdatacollectedovertimearenecessarytoquantifychangesinaquiferstorageandtoevaluatetheseasonalvariabilityofaquiferrecharge.
PilotStudyRecommendations
ItishighlyrecommendedthatMPWMDinstallautonomouswaterlevelmonitorsinselectedwellswithinboththelow‐andhigh‐scoringregionsofthepilotstudy.Thesesimpleandrobustinstrumentswillprovidecontinuouswaterleveldatathatarecriticaltoquantifyingtemporalandspatialtrendsinaquiferdrawdownandrecharge,thusprovidingthecriticalelementnecessarytoevaluatefracturedrockaquiferreliabilityandsustainability.
Atthelocalwellscale(~1,000ft2),completebedrockmappinginconjunctionwithfractureanalysiswillprovideusefulinformationonpreferentialgroundwaterflowpathsandhelpdiscriminatebetweencompetingmodelsoffluidflow.Forexample,productionfromwellsscreenedinhostrockwithnumeroussmalldiscontinuousfractureswilldiffersignificantlyfromthosescreenedacrosslargecontinuousfractures.
Attheregionalscale(1‐100mi2),lineamentanalysisshouldbeusedtoidentifyregionalbedrockfabricandstructurethatmaydirectlyinfluencegroundwaterflowpatterns.
DistrictWideRecommendations
ItishighlyrecommendedthatMPWMDundertakeasimilareffortinallfracturedrockregionsoftheDistrictinordertoevaluatethereliabilityandsustainabilitytheseaquifers.Thiseffortshouldbuilduponthestructureofthepilotstudyandincludetherecommendationscitedabove.
Additionally,ongoingwaterlevelmonitoringrequirementsshouldbeaddedtotheWaterDistributionSystemProcess.Thiswillaidinquantifyingthetemporalchanges(bothseasonalanddecadal)ingroundwaterlevelsandyieldcriticalinsightintothesustainabilityofthesefracturedrocksystems.
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Figure 1. Production history from 1992 – 2008 for 38 wells with production data from the 68 wells identified within the pilot study. Production has increased significantly from 6 wells producing ~5 acre‐feet/year in 2000 to 35 wells producing ~30 acre‐feet/year in 2008.
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Figure 2. Drawdown ratios (DD) for wells screened in Monterey Shale and fractured granite within the pilot study regions. Ratios are typically higher for wells screened in the shale, suggesting that these wells are more efficient in groundwater extraction.