Assessing Structural geological controls on groundwater processes in mountain settings: Insights from three?dimensional numerical modeling

Abstract

Mountainsplayacriticalroleinthehydrologicalcyclebytransferringheavyprecipitationtolowlandaquifers.However, theircomplexityandremotenesslimitourunderstandingofgroundwaterflow,particularlytheinfluenceoffaults.Tofill thegap,semi?idealized3Dnumericalmodelscalibratedusingthemountainrivernetworkandthelowlandpiezometricgradientweredeveloped.Theimpactoffaultsongroundwaterflowwasexploredbyvaryingtheirhydraulicconductivity,position,orientation,andlength.Themetricsevaluatedwereflowpartitioning,seepagearea,flowpathlengths,andresidencetimes.Itwasfoundthatthehydraulicconductivitycontrastbetweenafaultandthepervasiverockcontrolsrechargepartitioningasmuchastheoverall transmissivityofthepervasiverock.Regionalconductivefaultsparallel totheorogenpromotemountain?blockrechargeoversurfaceflow,assignificantlyasthicksystemsdo,andviceversa.Localscalefaultscanexertasmuchinfluenceasregionalfaultswhencrossingthecatchmentoutlet,highlightingtheimportanceoflocalheterogeneityinregionalflowdynamics.Intercatchmentflowisprimarilygovernedbylithologyandtopographyandismodulatedbythefaultpositionrelativetomajortopographicfeatures.Faultsinfluenceseepageareaswithinamulti?kilometerdistanceincharacteristicpatternsusefulforsegregatingtheireffectiverole.Byloweringthewatertable,conductivefaultssystematicallyreducetheseepageareas.Meanwhile,barriersdecreaseseepageareasdownstreamoftheirtraceandincreasethemupstream,withoutaffectingtheextentofseepage.Finally, thedistributionsofflowpathlengthsandresidencetimesareuncorrelated,highlightingtheimportanceofnumericalmodelingforgroundwaterdating