John A. Welhan , Affiliate Faculty & Research Hydrologist with The Idaho Geological Survey
Email: email@example.com - - - Phone: 208-282-4254
- Geostatistics and stochastic modeling
- Aqueous and Environmental isotope geochemistry
- Ph.D., 1981, University of California at San Diego, Scripps Institution of Oceanography (isotope geochemistry)
- M.Sc., 1974, University of Waterloo, Canada (hydrogeology)
- B.Sc., 1972, University of Manitoba(Geology)
- Joined the Idaho State University faculty in 1990
Who Am I ?
Idaho Geological Survey, Pocatello Branch Office supervisor; Full Research Geologist, University of Idaho; Affiliate Faculty, Idaho State University
Undergrad Courses I Teach:
Geostatistics and Spatial Modeling is taught in Spring semester, in odd-numbered years.
- Geostatistical modeling of groundwater monitoring data from large networks in both spatial and temporal coordinates.
- Strochastic modeling of heterogeneity and characterization of basalt morphology and its control on preferential ground water flow and contaminant transport in the Snake River Plain aquifer.
- Geologic mapping of subsurface lithology and stratigraphy in the eastern Snake River Plain with well drillers' logs.
- Characterizing the impact of agricultural activities on shallow ground water chemistry.
- Monitoring and modeling the impact of septic leachate on shallow ground water flow systems .
Welhan, J.A., R.L. Farabaugh , Merrick , M.J. and S.R. Anderson , in press . Geostatistical Modeling of Sediment Abundance in a Heterogeneous Basalt Aquifer at the Idaho National Laboratory, Idaho ; U.S. Geological Survey Scientific Investigations Report.
Basagaoglu, H., P. Meakin, S. Succi and J. Welhan, 2005. Boundary effects on the onset of nonlinear flow in porous domains; Europhysics Letters, vol. 73(6), pp. 1-6.
Welhan, J.A., C.M. Johannesen, L.L. Davis, K. Reeves, and J.A. Glover , 2004 . Overview and synthesis of lithologic controls on aquifer heterogeneity in the eastern Snake River Plain, Idaho; in Bill Bonnichsen, C.M. White, and Michael McCurry, eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 435-460.
Welhan, J.A. and Merrick. M. , 2004. Statewide Network Data Analysis and Kriging Project, Final Report; Idaho Department of Water Resources on-line report; http://www.idwr.idaho.gov/hydrologic/info/pubs/gwq/IGS_Kriging_Project-Final_Report.pdf.
Welhan, J.A., T. Clemo and E. Gego, 2002. Stochastic simulation of aquifer heterogeneity in a layered basalt aquifer system, eastern Snake River Plain, Idaho, in Link, P.K., Mink, R. and Ralston, D., eds., Geology, hydrogeology and environmental remediation, Idaho National engineering and Environmental Laboratory, eastern Snake River Plain, Idaho; Geological Society of America, Special Paper 353,p. 225-247.
Welhan, J.A., C.M. Johannesen, K. Reeves, T. Clemo, J.A. Glover and K. Bosworth , 2002 . Morphology of inflated pahoehoe lavas and spatial architecture of their porous and permeable zones, eastern Snake River Plain, Idaho, in Link, P.K., Mink, R. and Ralston, D., eds., Geology, hydrogeology and environmental remediation, Idaho National Engineering and Environmental Laboratory, eastern Snake River Plain, Idaho; Geological Society of America, Special Paper 353, p. 135-150.
Gego, E.L., G.S. Johnson, M.R. Hankin, J.A. Welhan , 2002. Modeling ground water flow and transport in the Snake River Plain aquifer: a stochastic approach; in Link, P.K., Mink, R. and Ralston, D., eds., Geology, hydrogeology and environmental remediation, Idaho National engineering and Environmental Laboratory, eastern Snake River Plain, Idaho; Geological Society of America, Special Paper 353.
Welhan, J.A., 2001. Water quality in the Portneuf aquifer, Pocatello area, Idaho : the case for protection; Journal of the Idaho Academy of Sciences, v. 37, p. 88-92.
Cecil, L.D., J.A. Welhan, J.R. Green, S.K. Frape, and E.R. Sudicky, 2000. Use of chlorine-36 to determine regional-scale aquifer dispersivity, eastern Snake River Plain aquifer, Idaho; Nuclear Instruments and Methods in Physics Research B 172, p. 679-687.
Clemo, T. and J. Welhan, 2000 . Simulating basalt lava flows usings a structure imitation approach, in Bentley, L.R., Sykes, J.F. and Brebbia, C.A. (eds), Computational Methods in Water Resources, Proc. XIIIth International Conf. on Computational Methods in Water Resources, p.841-848, Balkema, Rotterdam.
Welhan, J.A. and Reed, M. 1997. Geostatistical analysis of regional hydraulic conductivity variations in the Snake River Plain aquifer, eastern Idaho; Bull. Geol. Soc. America, v.109, pp. 855-868.
Welhan, J.A., Smith. R.P. and Wylie, A. 1997. Stochastic modeling of hydraulic conductivity in the Snake River Plain aquifer: 1. Hydrogeologic constraints and conceptual approach; Proc. 32nd Symp. Eng. Geology and Geological Engineering, Boise, pp. 75-92.
Welhan, J.A. and Wylie, A. 1997. Stochastic modeling of hydraulic conductivity in the Snake River Plain aquifer: 2. Evaluation of lithologic controls at the core and borehole scales; Proc. 32nd Symp. Eng. Geology and Geological Engineering, Boise, pp. 93-107. Welhan,
Welhan, J.A., Meehan, C. and Reid, T.V. 1996. The lower Portneuf River valley aquifer: a geologic and hydrologic model, and implications for wellhead protection strategies; Final Report, EPA Wellhead Protection Demonstration Project and City of Pocatello Aquifer Geologic Characterization Project, 131 pp.
Reid, T.V., Welhan, J.A. and Pelton, J.R. 1995. Gravity-defined subsurface structure and geometry of the Lower Portneuf River aquifer; Proc. 31st Eng. Geology and Geological Engineering Symp., Logan, Utah, pp. 429-442.
Welhan, J. and Meehan, C. 1994. Hydrogeology of the Pocatello aquifer: implications for well head protection strategies; Proc., 30th Eng. Geology and Geological Engineering Symposium, Boise, March 23-25, 1994, pp. 1-18.
Welhan, J.A., Fromm, J. and McCurry, M. 1992. Application of ground water tracer methods in straddle packer testing at the ICPP, Idaho National Engineering Laboratory, Proc., Ground Water Quality Technical Workshop, Boise, 1992, pp. 16-26.
Selected Recent Abstracts
Geostatistical Modeling of Sediment Abundance in a Heterogeneous Basalt Aquifer at the National Laboratory, Idaho; U.S. Geological Survey Scientific Investigations Report. J.A. Welhan, R.L. Farabaugh, M.J. Merrick, and S.R. Anderson , in press.
The spatial distribution of sediment in the eastern Snake River Plain aquifer was evaluated and modeled to improve the parameterization of hydraulic conductivity (K) for a subregional-scale ground-water flow model being developed by the U.S. Geological Survey. The aquifer is hosted within a layered series of permeable basalts within which intercalated beds of fine-grained sediment constitute local aquitards. These sediments have K values as much as six orders of magnitude lower than the most permeable basalt, and previous flow-model calibrations have shown that hydraulic conductivity is sensitive to the proportion of intercalated sediment.
Stratigraphic data in the form of sediment thicknesses from 333 boreholes in and around the Idaho National Laboratory (INL) were evaluated as grouped subsets of lithologic units (composite units) corresponding to their relative time-stratigraphic position. The results indicate that median sediment abundances of the stratigraphic units below the water table are statistically invariant (stationary) in a spatial sense and provide evidence of stationarity across geologic time, as well. On the basis of these results, borehole data were kriged as two-dimensional spatial data sets representing the sediment content of the layers that discretize the ground-water flow model in the upper 300 feet of the aquifer.
Multiple indicator kriging (mIK) was used to model the geographic distribution of median sediment abundance within each layer by defining the local cumulative frequency distribution (CFD) of sediment via indicator variograms defined at multiple thresholds. The mIK approach is superior to ordinary kriging because it provides a statistically best estimate of sediment abundance (the local median) drawn from the distribution of local borehole data, independent of any assumption of normality. A methodology is proposed for delineating and constraining the assignment of hydraulic conductivity zones for parameter estimation, based on the locally estimated CFDs and relative kriging uncertainty. A kriging-based methodology improves the spatial resolution of hydraulic property zones that can be considered during parameter estimation and should improve calibration performance and sensitivity by more accurately reflecting the nuances of sediment distribution within the aquifer.
Spatial-Temporal Geostatistics for Ground Water Quality Networks, Inland Northwest Research Alliance, Subsurface Science Symposium, Spokane ; http://www.inra.org/INRA%20NEWS,%20EVENTS%20PAGES/ESSS04%20Agenda.htm , J.A. Welhan and T. Masiane, 2004.
Variography, kriging, and stochastic simulation have been widely applied in the analysis and synthesis of ground water quality data: to understand spatial relationships, to model large-scale patterns of variation, to quantify local uncertainty for cost and risk assessment, and to map areas of statistically-significant change. For example, 2D indicator kriging can be used to map exceedance probability (e.g., over what area is it more than 90% probable that nitrate-N exceeds 5 mg/l?); multiple such maps delineate areas of chronic exceedance or exceedance frequency (where and how often is the 90% probability level exceeded?); and 2D estimation variance maps
can be coupled with the geostatistical estimates to identify areas of temporal change that are statistically significant or insignificant at a specified confidence level.
Spatial-temporal (ST) geostatistics holds even greater promise for the synthesis of multi-year data sets collected from regional and/or local monitoring networks. ST geostatistics considers both spatial and temporal autocorrelation to reduce the uncertainty of local temporal change estimates by exploiting the similarity (temporal persistence) of water quality measurements over time, in the same and nearby wells. In effect, water quality information from multiple sampling events is used to create a 3D space-time "map" of how water quality varies in geographic space over the life of the network. Such maps can be constructed using either kriging or stochastic simulation techniques, depending on the goals of the project. In all cases, estimates of the spatial and temporal variation of analyte concentration are accompanied by estimates of local estimation uncertainty which can be used to quantify the level of statistical confidence in the estimates and their changes over time.
Eleven years of ground water nitrate data from the Treasure Valley, derived from Idaho's Statewide Ground Water Quality Monitoring Network, and from the Idaho Falls area, derived from the Idaho Department of Water Quality's community water monitoring system, are used to show how geostatistical estimation can be improved by ST geostatistical analyis, how confidence in temporal change assessment can be improved, and how chronic exceedance maps can be constructed more accurately by analyzing coupled spatial and temporal change in water quality.
Overview and synthesis of lithologic controls on aquifer heterogeneity in the eastern Snake River Plain , Idaho ; Idaho Geological Survey Bulletin 30, p. 435-460. J.A. Welhan, C.M. Johannesen, L.L. Davis, K. Reeves, and J.A. Glover , 2004 .
The factors controlling basalt permeability and the heterogeneous distribution of permeability in eastern Snake River Plain (SRP) basalt are reviewed and new information summarized. A refined conceptual model of the lithologic architecture has been created on which stochastic models of aquifer heterogeneity and ground-water flow are being based. Basalt lava flow geometry and internal structures in drill core and in Holocene flow groups are remarkably similar. This suggests that the emplacement style of monogenetic lava flows in the eastern SRP has remained unchanged since the mid-Quaternary, so that Holocene basalts can be used as analogs of the subsurface to identify the features controlling heterogeneity of permeability.
On the basis of core data, inflated pahoehoe lava appears to represent more than 80 percent of eastern SRP basalts. This suggests that the inflated pahoehoe morphology in distal zones of Holocene flow groups is the most volumetrically important basalt morphology of SRP shield volcanoes. Aerial photo and field mapping of large-scale, inflated lava flow units in the Wapi and Hell's Half Acre flow groups has shown these structures to be characterized by a fractal shape that reflects their growth process. Because of fractal scaling, information on large-scale features may be extrapolated to smaller flow lobes. Length to width aspect ratios of lava flow lobes can exceed 10 : 1 but median aspect ratios are close to 3 : 1 and remarkably similar in different Holocene flow groups.
Our conceptual hydrogeologic model is based on three principal permeability-controlling lithologies: low-permeability sediments, low-permeability massive basalt, and high-permeability zones within basalt. Two types of potential high-permeability zones have been identified in inflated pahoehoe lava flows: Type-1 interflow zones of highly porous, rubbly and broken material at the contacts between lava flow lobes; and Type-2 networks of fissures which can remain partially open after burial by younger lava. The spatial continuity of interflow zones is expected to be constrained by the dimensions and geometry of flow lobes. If fissures remain sufficiently permeable and interconnected after burial, their spatial continuity could approach kilometer scales and be highly directional. Open-interval permeabilities in individual wells correlate with the number of high-porosity zones (potentially permeable zones) intersected by the well. The large amount of scatter about the correlation suggests that the permeability of individual high-porosity zones are highly variable and that the permeability of any given high-porosity zone will be a stochastic variable.
Probability mapping as a decision tool for ground water quality monitoring; 14th Nonpoint Source Water Quality Monitoring Workshop, Boise; Abstracts with Programs, p.9-10, J. Welhan, M. Merrick, K. Neely and E. Hagen, 2004.
Resource monitoring is an important tool for formulating management policy where competing resource interests exist. However, if monitoring data are underutilized and/or inadeqately communicated to users, then monitoring's effectiveness as well as its political currency both erode. Idaho 's Statewide Ground Water Quality Monitoring Network, a database with 11 years of data on over 400 analytes from 1800 wells and springs statewide, contains a wealth of information to support decisions regarding ground water management. As scientists, we can assist the policy process by developing tools to analyze this data in a manner that policy makers can understand and apply. The ideal decision-making tools need not be quantitative, necessarily, but must be objective and defensible, and they should provide a measure of uncertainty or risk that can be incorporated in policy decisions that weigh competing needs.
Kriging is known as an optimal spatial interpolator that honors all measurements and minimizes estimation uncertainty. It is ideally suited to portraying monitoring information in map form that can be used by decision makers. For example, kriging can be used to produce maps of the likelihood that water quality exceeds a specific threshold (e.g., where is it 90% probable that nitrate exceeds 5 mg/l?). Such exceedance probability maps can be constructed across the life of a monitoring program and summarized in a single map of exceedance frequency (e.g., where is the 90% probability level most often exceeded?). Using nitrate data from the Statewide Network, we show how areas of degraded water quality can be identified via probability mapping, and compare these to subjectively-delineated areas of concern which Idaho regulatory agencies currently use. Probability kriging has the advantages of being quantitative, easy to understand and defend, and statistically rigorous. Furthermore, delineated areas can be regularly updated as new monitoring data accrue.
Stochastic simulation of aquifer heterogeneity in a layered basalt aquifer system, eastern Snake River Plain, Idaho; Geological Society of America, Special Paper 353, p. 225-247. Welhan, J.A., T. Clemo and E. Gego, 2002.
A stochastic approach to modeling aquifer heterogeneity in basalt lava flows was evaluated using different methods. Direct simulation of permeability was not justified with the limited amount of permeability data available. The lithology was interpreted from borehole geophysical logs, and three lithologic categories were used as permeability surrogates: massive basalt and fine-grained sediment, both of low permeability, and high-permeability interflow zones between lava flows.
Sequential multiple indicator simulation of lithology was the first modeling method evaluated. Indicator semivariograms of lithology were described with a nested model. The horizontal ranges of the sediment and massive basalt categories were 140 m and 150 m, respectively, and isotropic. The maximum horizontal range of the interflow zones was 110 m, with a horizontal range anisotropy of 7:1, elongated east-west. A 1.74 km 2 by 60-m-thick volume of aquifer was discretized on a 6 ´ 6 ´ 0.6 m grid and simulations were conditioned to borehole lithology. A major limitation of this approach is that permeabilities assigned to the simulated lithologies are not conditional to available well-test permeability data.
The second approach was simulated annealing of well-scale permeability, conditioned to limited well-test data and to previously simulated lithology, with a vertical resolution matching the average 6 m measurement support of well-test data. A relationship was demonstrated between well-test permeability ( K b ) and number of interflow zones, N Z , in the well-test interval, thereby allowing both lithologic data and simulated lithologic structure to be used as additional conditioning information. The K b - N Z relationship indicated that interflow zone permeability is lognormal with a relatively small variance.
A preliminary structure-imitating model was developed to investigate the feasibility of a variant of an object-based simulation approach. The model mimics the physical geologic structure created by processes such as eruption, inflation, ponding, and sedimentation, by optimizing specified rules and parameters that produce stochastic realizations which reproduce the morphologic and geometric characteristics of lava flows.
Welhan, J.A. and Reed, M.F., Geostatistical Analysis of Regional Hydraulic Conductivity Variations in the Snake River Plain Aquifer, Eastern Idaho Bull. Geol. Soc. America, v. 109, pp. 855-868. J.A. Welhan and M.F. Reed, 1997.
The regional spatial correlation structure of bulk horizontal hydraulic conductivity (Kb), estimated from published transmissivity data from 79 open boreholes in the fractured basalt aquifer of the eastern Snake River Plain at the Idaho National Engineering Laboratory, was analyzed with geostatistical methods. The two-dimensional spatial correlation structure of ln(K b ) shows a pronounced 4:1 range anisotropy, with a maximum correlation range in the NNW-SSE direction of about 6 km. The maximum variogram range of ln(K b ) is similar to the mean length of flow groups exposed at the surface and the ln(K b ) range anisotropy is similar to the mean width/length ratio of late Quaternary and Holocene basalt lava flows, and the orientations of the major volcanic structural features on the eastern Snake River Plain. The similarity between lnKb correlation scales and basalt flow dimensions and between basalt flow orientations and correlation range anisotropy suggests that the spatial distribution of zones of high hydraulic conductivity may be controlled by the lateral dimensions, spatial distribution and interconnection between highly permeable zones which are known to occur between lava flows within flow groups. If hydraulic conductivity and lithology are eventually shown to be cross-correlative in this geologic setting, it may be possible to stochastically simulate hydraulic conductivity distributions which are conditioned on a knowledge of volcanic stratigraphy.
Stochastic Modeling of Hydraulic Conductivity in the Snake River Plain Aquifer: 1. Hydrogeologic Constraints and Conceptual Approach Proc. 32nd Symp. Eng. Geology and Geological Engineering, Boise, pp. 75-92. J. Welhan, T. Funderberg, R.P. Smith and A. Wylie
The transport of ground water and solutes in the Snake River Plain aquifer beneath the Idaho National Engineering and Environmental Laboratory (INEEL) is dominated by the presence of localized ground water flow zones characterized by extremely high hydraulic conductivity. The principal problem in flow and transport modeling at the INEEL is in describing the spatial variability of hydraulic conductivity in a volcanic-sedimentary lithologic environment in which hydraulic conductivity varies by six orders of magnitude over decimeter scales. The goal of the modeling effort is to describe the 3-D spatial distribution and continuity of high-conductivity zones that tend to be located at lava flow margins. Geostatistical interpolation and stochastic modeling techniques have been shown to be powerful tools for describing subsurface heterogeneity, but the nature and scale of the stochastic modeling problem must be posed carefully. The framework for developing a stochastic model is based on a scale-dependent view of subsurface heterogeneity. To clarify scale-dependent terminology of basalt lava flow groups, the term Supergroup is proposed to describe a monogenetic collection of subsurface basalt flow groups derived from several volcanic vent sources. Due to the relative paucity of hydraulic conductivity data, lithologic surrogates of high- and lowconductivity geologic materials will be modeled, with available hydraulic data acting to constrain the stochastic simulations. The stochastic modeling approach is built on: 1) the concept of preferential flow zones developed along lava flow margins and a statistical correlation between borehole hydraulic conductivity and the number of lava flow contacts; 2) a statistical comparison of the geometries of basalt flow elements and flow units of different ages demonstrating that basalt flow architecture has not changed through the Pleistocene and Holocene, so that 3) the geographic distribution and spatial continuity of Holocene basalt lava flows can be used as an analog for the horizontal spatial continuity of lava flows and thus of high-conductivity zones at depth; 4) the use of formation density logs as a tool to map highconductivity lithologic zones in the subsurface; and 5) the use of indicator statistics to describe and model the spatial distribution of extreme-valued hydraulic conductivity zones.
Johanneson, C., Davis, L., Funderberg, T., and Welhan, J. (September 1997) - Basalt Surface Morphology as a Constraint on Lateral Spatial Correlation Structure in Stochastic Simulations of Hydraulic Conductivity in the Snake River Plain Aquifer. Proc. Symposium Ground Water in the Rocky Mountain Region.
The stochastic simulation of the 3-D hydraulic conductivity field in the Snake River Plain (SRP) aquifer cannot proceed on the basis of borehole data alone, because of the difficulty in evaluating inter-borehole spatial correlation characteristics from widely-spaced boreholes. This work is built on the hypothesis that Holocene basalts exposed at the surface of the SRP serve as analogs of buried basalts whose internal structure and geometry does not vary over time (Welhan et al, 1997). Since high-conductivity zones are concentrated at the margins of SRP basalt flow units, mapping of the areal geometry and orientations of lava flow units was proposed as a means to constrain the horizontal spatial correlation structure of these preferential flow zones (Welhan et al, 1997).
Several lines of evidence are being pursued in order to facilitate large-scale mapping of relevant basalt morphologic features on the SRP: 1) spatial correlation analysis of existing morphologic information that has been mapped at the local and the regional scales; 2) the analysis of aerial color imagery to enhance detection of relevant morphologic features; and 3) the evaluation of high-resolution, airborne and satellite synthetic aperture radar imagery as a means of detecting large-scale surface morphologic structure.
Mapping of the lengths, aspect ratios and areas of basalt flow units will constrain the horizontal correlation scales of the associated high-conductivity lava flow margins, through the application of threshold-crossing theory (Phillips and Wilson, 1989). Conditional indicator simulation will utilize this horizontal correlation structural information, together with vertical structural information gleaned from analysis of borehole data, to generate stochastic realizations of the 3-D distribution of high-conductivity zones in the SRP. These realizations, in turn, will serve as input to a stochastic ground water flow model to evaluate the effect of heterogeneity on the ground water flow field.
Glover, J., Davis, L., and Welhan, J. (Sepember 1997) - The Use of Borehole Geophysical Logs and Geologic Data for Stochastic Simulation of Hydraulic Conductivity in the Snake River Plain Aquifer. Proc. Symposium Ground Water in the Rocky Mountain Region, Boise.
The approach being taken to stochastically simulate the 3-D spatial distribution of preferred ground water flow zones in the Snake River Plain (SRP) aquifer exploits the correlation between bulk hydraulic conductivity and the vertical spatial density of lava flow margins within a borehole test interval that has been observed (Welhan et al, 1997). Stochastic simulations will rely on abundant "soft" geologic information to constrain the spatial distribution of high- and low-conductivity zones in basalt. We are developing an interpretational protocol for borehole geophysical logs so that the spatial locations of lava flow margins and other hydraulicallyrelevant features in SRP boreholes can be mapped. We are focusing on four major types of hydraulically-relevant lithologic features in the SRP basalts which are associated with preferential flow zones. In decreasing order of relative hydraulic conductivity, these are: 1) rubbly, broken zones occurring at lava flow margins; 2) vertically-fractured intervals in massive basalt; 3) highly-porous, vesiculated zones above and below non-rubbly lava flow margins; and 4) low permeability counterparts of the above where partial sediment infilling occurs.
The borehole logs of greatest use for this purpose are the density (induced gamma), porosity (neutron), borehole diameter (caliper) and natural gamma logs. Work is progressing on identifying the geophysical log responses that are characteristic of the above lithologic features, as well as uniformly conditioning the available geophysical log data in order to reduce interpretational bias from hole to hole.
The database created from the geophysical log interpretations will yield a measure of the spatial density of preferential flow zones (Fp), which will be correlated with available borehole hydraulic conductivity data. The Fp dataset, together with borehole data on sedimentary interbed occurrences, will constitute the conditioning data for sequential indicator simulation of high- and low-conductivity zones in the SRP aquifer. These stochastic realizations of the hydraulic conductivity field will serve as input to a stochastic ground water flow model for assessing the impact of heterogeneity on the three-dimensional flow field.
Welhan, J.A. and Reed, M.F., Geostatistical Analysis of Regional Hydraulic Conductivity Variations in the Snake River Plain Aquifer, Eastern Idaho Bull. Geol. Soc. America, v. 109, pp. 855-868.Welhan, J.A. and Reed, M.F.
The regional spatial correlation structure of bulk horizontal hydraulic conductivity (Kb), estimated from published transmissivity data from 79 open boreholes in the fractured basalt aquifer of the eastern Snake River Plain at the Idaho National Engineering Laboratory, was analyzed with geostatistical methods. The two-dimensional spatial correlation structure of lnKb shows a pronounced 4:1 range anisotropy, with a maximum correlation range in the NNW-SSE direction of about 6 km. The maximum variogram range of lnKb is similar to the mean length of flow groups exposed at the surface and the lnKb range anisotropy is similar to the mean width/length ratio of late Quaternary and Holocene basalt lava flows, and the orientations of the major volcanic structural features on the eastern Snake River Plain. The similarity between lnKb correlation scales and basalt flow dimensions and between basalt flow orientations and correlation range anisotropy suggests that the spatial distribution of zones of high hydraulic conductivity may be controlled by the lateral dimensions, spatial distribution and interconnection between highly permeable zones which are known to occur between lava flows within flow groups. If hydraulic conductivity and lithology are eventually shown to be cross-correlative in this geologic setting, it may be possible to stochastically simulate hydraulic conductivity distributions which are conditioned on a knowledge of volcanic stratigraphy.
Welhan, J., Meehan, C., & Reid, T. - The Lower Portneuf River Aquifer: A geologic / hydrologic model and its implications for wellhead protection strategies.
Excerpts from Executive Summary, Final Report for EPA Wellhead Protection Demonstration Project and the City of Pocatello Aquifer Geologic Characterization Project.
This report summarizes the findings of a multi-year study of the lower Portneuf River Valley (LPRV) aquifer system. The LPRV aquifer system is a unique ground water reservoir which is of critical importance to the cities of Pocatello and Chubbuck. The aquifer system comprises two very different sub-systems: the northern aquifer system and the southern aquifer. Based on recent gravity surveys, the northern aquifer sub-system appears to be several thousand feet deep, and is hydraulically confined by two or more aquitards.
Ground water in the southern aquifer recharges most of the northern aquifer system. The southern aquifer is a narrow, relatively shallow strip aquifer hosted in very permeable, coarse gravels, characterized by high linear flow velocities and physically separated from the northern aquifer by a prominent subsurface bedrock high. It appears to be lithologically unconfined but shows hydraulic indications of semi-confinement. The mean hydraulic conductivity estimated from wells in the southern aquifer is 2400 ft/d , with a range of 200-8200 ft/d. At an effective porosity of 0.3, ground water flow velocity is of the order of 5 - 25 ft/day.
Ground water flow from the Bannock Range and the Portneuf Gap are the principal sources of recharge to the LPRV aquifer system, with Bannock Range sources representing about 30% of the total recharge during 1994 (a dry year) and more than 50% during 1993 (an above-average water year). In terms of the magnitudes of water balance components, southern aquifer pumping needs were almost completely met by the amount of ground water that flowed into the LPRV through the Portneuf Gap during 1993/94. Similarly, the northern well field relied on outflow from the southern aquifer for 80% of water withdrawn in the northern aquifer. Total well field pumping withdrawal during the 1993/94 water balance accounting period represented 90% of the total known recharge to the LPRV aquifer, suggesting that the aquifer system may be approaching its maximum safe yield during sustained drought conditions such as have existed for the eight years prior to 1993.
Analytically-modeled capture zones defined for the southern aquifer production wells are considered to be very coarse approximations of true capture zone geometry. Effective delineation of capture zones should be accomplished with a numerical flow and particle-tracking model. The prospect for developing individual WHP areas for 5-year time of travel that are both manageable and technically defensible appears slim. It is recommended, instead, that emphasis be focused on defining basin-wide WHP Recharge Zones defined by hydrogeologic boundaries.
Of the aquifer's principle recharge source areas, the Bannock Range / Mink Creek Recharge Zone has the greatest potential for development of a workable, basin-wide WHP plan. The area is of manageable size, an historic precedent exists in the exclusion of part of the City Creek drainage for water supply protection, and land-use management guidelines can be developed more readily in this mostly unpopulated, forested area than in populated areas. Development of a cooperative plan with the US Forest Service for management of contiguous lands may provide mutual benefits to the participating agencies.
Ground water underflow through the Portneuf Gap originates from a huge recharge source area (i.e.. the upper Portneuf River basin) that is probably unmanageable because of its geographic size, the diversity of land uses within that area and the potential for political conflict between agricultural and urban water users. However, the Portneuf Gap is a highly manageable inflow zone, a geographically well-defined area through which all up-gradient flow passes. The area immediately up-gradient of the Portneuf Gap provides a significant buffer zone between upper basin inputs and the immediate jurisdictional boundaries of the LPRV aquifer. It could represent a useful monitoring zone in which observation wells could provide early-warning capability for developing water quality problems.