Dale R. Ralston

Groundwater response to snowmelt in a mountainous watershed

Abstract Snowmelt recharge to shallow groundwater systems is the primary source of streamflow in many mountainous watersheds, but characteristics of these systems are not well understood, and their contribution to streamflow is often not appreciated. Data from a detailed study on the Upper Sheep Creek Watershed located within the Reynolds Creek Experimental Watershed in southwestern Idaho were analyzed to characterize the interactions between snowmelt, groundwater and streamflow. Response time between snowmelt, groundwater levels and streamflow was drastically different from year to year depending on the extent of the snowpack. Response time to snowmelt for piezometer and weirs located 135 m downslope from am isolated drift was 3–5 days during an average snow year and up to 70 days for a year with snow accumulation that was 40% of normal. The primary aquifer is believed to be unconfined during low snowmelt years and confined when normal or above-normal snowmelt causes high groundwater levels. Snowmelt from an isolated drift enters the primary aquifer upslope of the confining layer. Rapid response during years with normal snow accumulation is therefore primarily a pressure pulse through the confined aquifer. Recharge during years of low snow accumulation is insufficient to fill the primary aquifer to the confining layer,and response time is indicative of travel time through the aquifer.

Hydrogeologic role of geologic structures. Part 1: the paradigm

Abstract Grouting to reduce fracture permeability is one option for minimizing ground water inflow to a large, acid-producing lead-zinc mine in fractured metamorphic rock in north Idaho. For grouting to reduce mine water inflow effectively, the hydrogeologic characteristics of the various scales of structurally controlled fracturing must be identified and a conceptual model of the ground water flow system must be developed. This paper is the first of two papers which use fracture mapping, geologic structural mapping, and a series of underground hydraulic stress tests to develop a conceptual model of structurally controlled ground water flow in the vicinity of the mine. These data were collected in an effort to identify order within the structurally controlled spatial permeability distribution. Geologic structural discontinuities, ranging from joints to faults that extend for several miles, form a geologic structural hierarchy in the rock surrounding the mine. The tiers of the hierarchy control ground water flow into the mine at different scales. The most prominent faults control ground water inflow on the scale of the entire mine. Various levels of hydraulic continuity are evident within the rock mass bounded by two of the most prominent faults. The highest level of hydraulic continuity appears to be associated with a set of sub-parallel, steeply dipping faults. Minor faults, joints, and relict bedding planes to a limited extent connect the fractures of this set and form a lower level of hydraulic continuity. The next lower level of hydraulic continuity within the hierarchy is related to a major fault that is characterized in drill core by abundant gouge. The hydraulic continuity of the matrix within the unfractured quartzite is the lowest level within the hierarchy. These levels constitute the components of the order within the spatial permeability distribution that we have interpreted from the structural and hydraulic stress test data.

Hydrogeochemistry of the Meade thrust allochthon, southeastern Idaho, U.S.A., and its relevance to stratigraphic and structural groundwater flow control

Abstract Groundwater flow patterns in the folded carbonate, sandy and shaly strata of the Meade thrust allochthon, southeastern Idaho, U.S.A., were investigated using classical geochemical and isotopic techniques. Samples from 38 springs and wells were analyzed for major ions, δ 18 O and δ 2 H, and samples from nine springs were analyzed for radiocarbon content and δ 13 C. Two major groundwater regimes were identified. Aquifers of the upper regime (tier one) are restricted to strata above the Phosphoria Formation. Their waters circulate within a few hundred meters of the surface. Discharge waters are of recent meteoric origin, having temperatures of 12°C or less, and are generally undersaturated with respect to calcite, and have Ca/Mg of ∼ 2.5. Groundwater flow is generally bedding plane controlled and discharge is along bedding and thrust splay surfaces. No indication of significant groundwater circulation between this and underlying strata (tier two) was found. Aquifers of the lower regime (tier two) are restricted to consolidated strata below the Phosphoria Formation which generally hydraulically isolates them from tier-one aquifers. Groundwater flow in tier two circulates as much as 1900 m below land surface. Discharge waters have high temperatures up to 26°C. Stable-isotopic data suggest the temperatures have not been elevated above 100°C. The discharge waters are saturated with respect to calcite, have Ca/Mg of ∼ 3.4 and have mean subsurface residence times of 12,500–20,500 yr. estimated from radiocarbon data. These spring waters are evolving CO 2(g) , possibly from the upward diffusion along faults, and have deposited massive amounts of travertine. Groundwater flow is generally bedding plane controlled, and discharge is along deep-seated extension and thrust faults. Based on maximum circulation depth and estimated depth of the Meade thrust fault, tier-two aquifers appear to be contained within the allochthon, with no evidence found suggesting they communicate with deeper circulating systems (tier three).

The hydrogeology of an underground lead-zinc mine: Water flow and quality characteristics

Acid mine drainage is a major environmental and economic problem. The abandonment of acid-producing mines is of particular concern. The University of Idaho is conducting a detailed investigation of the Bunker Hill Mine, in north Idaho, to determine the mechanisms controlling mine recharge and acid production and their impact on abandonment alternatives.The Bunker Hill Mine disturbs 20 cubic kilometers of highly fractured quartzites of the Revett and St. Regis formations (Belt Supergroup). The mine comprises 240 km of tunnels and drifts and 9.6 km of major inclined shafts, raises and winzes. The mine has 31 levels extending nearly 1830 m in depth. An average of 190 1/s of water is discharged from the mine. The discharge typically has pH about 3.0 and zinc concentrations in the 100 milligrams per liter range. Gravity drainage accounts for about 30 percent of the flow. Pumpage from lower levels accounts for the balance.Recharge to the mine occurs from fractures, exploration drill holes, and areas where mine workings extend to land surface. Understanding the temporal and spatial water flow and quality characteritics within the mine is essential to the design and abandonment procedures to mitigate long-term environmental and economic impacts.Research has been conducted underground in the Bunker Hill Mine since early 1983. Discharge/quality monitoring sites have been established to document both gravity drainage through the upper workings and pumpage from lower levels. Preliminary evaluation of data show differing temporal patterns of flow and quality within the mine. Data collection is continuing with specific emphasis on identifying specific areas and controls for recharge and the flow patterns in acid-producing areas of the mine.

Groundwater flow systems in the western phosphate field in Idaho

Abstract Ralston, D.R. and Williams, R.E., 1979. Groundwater flow systems in the western phosphate field in Idaho. In: W. Back and D.A. Stephenson (Guest-Editors), Contemporary Hydrogeology — The George Burke Maxey Memorial Volume. J. Hydrol., 43: 239-264. The complex geologic setting of the western phosphate field in Idaho provides the environment for equally complex groundwater flow systems. This research was initiated in 1974 to provide general and detailed hydrologic data on specific areas to aid in understanding the water-resource systems in the western phosphate field. Geologic, hydrogeologic and hydrologic data were collected in Little Long Valley and Lower Dry Valley in the Blackfoot River basin. Two groundwater flow systems are important in relation to the present and proposed mining in Little Long Valley: (1) the local shallow flow systems in the western ridge; and (2) the intermediate flow system in the Dinwoody Formation on the eastern ridge. The baseflow of Angus Creek in Little Long Valley is dependent on the groundwater flow systems in both ridges during the first one or two months following snow melt. The baseflow of the stream is almost completely dependent on the intermediate flow system in the eastern ridge for the rest of the year. Three groups of groundwater flow systems have been delineated in the Lower Dry Valley study area: (1) flow systems in the Thaynes and Dinwoody Formations along Schmid Ridge; (2) local flow systems in the shallow unconsolidated material on the east slope of Schmid Ridge; and (3) a Dry Valley—Slug Creek Valley flow system in the Wells Formation. The locations of the springs that discharge from the Thaynes and Dinwoody Formations are largely controlled by the axis of the Schmid Syncline and by small E—W-trending faults. The interbasin Dry Valley—Slug Creek Valley flow system is postulated based upon the geologic configuration formed by the Schmid Syncline, the loss of surface water on the floor of Dry Valley, the downward gradient in groundwater potential in the Phosphoria Formation in Lower Dry Valley and the existence and flow pattern of springs in the Slug Creek Valley. Care must be taken to avoid creating groundwater flow systems in any waste piles constructed as part of the mining activity. Drainage from a waste-pile flow system would probably be poorer quality than water from natural flow systems in the area.

Analytical modeling of a fracture zone in the Brule Formation as an aquifer receiving leakage from water-table and elastic aquitards

Abstract Hydrostratigraphic data define a subhorizontal, laterally limited, highly permeable zone of intensely fractured siltstone (major fracture zone) within the relatively impermeable Brule Formation at a site in Cheyenne County, Nebraska. Two types of transient hydraulic responses were noted in 11 observation wells during an eight-day aquifer test: Type 1 responses occurred in wells which intercepted the major fracture zone; Type 2 responses occurred in wells outside the major fracture zone. Transient responses in Type 1 observation wells are matched with type curves for a three-layered system with a water-table aquitard above an aquifer (major fracture zone) which overlies an elastic aquitard. Because the aquifer is laterally limited and the Brule Formation beyond the major fracture zone has finite hydraulic conductivity, real late-time behavior falls between type curves for an infinite aquifer and for an aquifer with an impermeable boundary at reasonable distances from the pumping well. Aquifer and aquitard parameters determined from curve matches with both limiting boundary conditions are identical for inner-ring observation wells, and nearly so for middle-ring observation wells. Outer-ring observation wells exhibited Type 2 responses which were used only to identify the extent of the major fracture zone.

SIMULATION OF ELECTRICAL POTENTIAL DIFFERENCES NEAR A CONTAMINANT PLUME EXCITED BY A POINT SOURCE OF CURRENT

Finite-difference simulations of electrical excitation of conductive contaminant plumes indicated that approximate dimensions of a plume and the approximate location of its center of mass can be derived, under specified circumstances, from the resulting electrical potential fields. Direct electrical excitation of a contaminant plume by a point current source was simulated for homogenous and isotropic conditions as well as in the presence of conductive clay layers and lenses. When a very shallow water table was assumed, changes in the electrical potential field between baseline (preplume) conditions and conditions that included a developing plume graphically formed a difference dipole. Simulations suggested that electrical flow is channeled preferentially through the negative difference pole at the approximate location of the center of mass in a dispersive contaminant plume. Electrical flow was channeled directly through the negative difference pole at the terminal end of a conductive clay lens. Simulations showed that even in the presence of conductive clays, the approximate location of the center of mass of an evolving contaminant plume could be delineated. This illustrates the potential future value of this approach, assuming continued technological advances in the field.