LetterReport:

FracturedRockAquiferPilotStudy

2010

Preparedfor:

JonathanLearMontereyPeninsulaWaterManagementDistrict

 

 

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

 

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

 

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.

 

 

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. 

 

 

 

 

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.