Water Balance and
Total Pumping Capacity of the Lower Portneuf River Valley Aquifer
By John Welhan, Idaho Geological Survey, Geology Department, Idaho State University. January, 2000. DRAFT
A detailed water balance of the upper part of the lower Portneuf River valley (LPRV) aquifer has been completed, in order to provide a best current estimate of available water capacity for build-out analysis. The LPRV aquifer relies on recharge form snowpack and precipitation in the southern Bannock Range for more than 70% of the total recharge required to sustain its water balance; less than 15% is derived from the upper Portneuf River basin through the Portneuf Gap and about 5% is estimated to derive from other drainages, principally the Pocatello Creek watershed. Current pumping withdrawals by the Cities of Pocatello and Chubbuck account for80-85% of this recharge, leaving approximately 1.1 to 1.5 billion gallons of annual capacity remaining, or approximately 14 - 17 % for future development.
Because we know very little of the water-bearing potential of the deeper part of the aquifer system not tapped by wells, we can only surmise that the impact on future capacity will depend on the depth and permeability at which deeper aquifers can be found, the quality of their ground water, and the rate at which they are recharged.
The potential to develop surface water runoff in the southern Bannock Range is limited. Even if as much as half the total surface runoff in southern Bannock Range watersheds could be diverted, it would increase total capacity by less than 5%.
This report was prepared in response to a request by the City of Pocatello's Community Development and Research Department to provide a best estimate of available water capacity for planning purposes. The City relies totally on grund water withdrawals from the lower Portneuf River valley aquifer for all its drinking, commercial and industrial water needs. The LPRV aquifer extends from the Portneuf Gap to Red Hill as a very narrow, strip aquifer; from Red Hill it widens and merges with the Snake River Plain aquifer to which it is tributary.
This report presents the results of the LPRV aquifer's water balance based on detailed information collected six years ago and supplemented with additional data and analysis. It focuses on the water capacity of the shallowest part of the ground water system that is currently exploited by wells. Because no wells have tapped deeper parts of the system, nothing is known about their water-bearing potential nor their ability to materially augment current water capacity.
Background and Previous Work
The LPRV aquifer system comprises two main parts: a northern aquifer system, from Red Hill northward, primarily confined and multi-layered; and a southern aquifer system, from the Portneuf Gap to Red Hill, in which only the most permeable, uppermost 100-150 feet of saturated thickness is currently exploited for domestic and municipal uses. Both northern and southern aquifers have several thousamd feet of deeper sediments that have not been tapped and about which almost nothing is known.
A detailed water balance of the LPRV aquifer in the southern part of the valley was completed as part of a comprehensive geologic and hydrologic characterization study of the LPRV (Welhan and others, 1996). The relevant section of that report is reproduced here as Appendix I. That water balance, covering a 510-day period in 1993 and 1994, is considered a highly accurate and representative estimate of the most hydrologically important part of the LPRV. It is considered accurate because of the well-defined bedrock geometry at the Portneuf Gap and at Red Hill which allowed ground water underflow through these important areas to be accurately calculated. It is also considered representative of typical water conditions because the 1993 water year was normal to above-normal, whereas the 1994 water year was a below-normal water year.
Figure 1 shows a summary of the results of the 1993/94 southern aquifer water balance. Principal sources of recharge to the aquifer were identified as the Mink Creek / Gibson Jack / City and Cusick Creek watersheds of the southern Bannock Range, at more than 5.5 billion gallons (Bgal) per year, and ground water underflow across the bedrock notch in the Portneuf Gap (about 1 Bgal annually). Negligible recharge is believed to originate from the Portneuf River and from the eastern side of the LPRV (Appendix I).
Ground Water Recharge to the Northern Aquifer
From the distribution of elevations in the LPRV watershed shown in Figure 2, the Pocatello Creek drainage is likely the only other besides the southern Bannock Range watersheds capable of supplying any significant recharge to the northern aquifer. However, no data or direct measurements exist to quantify Pocatello Creek's tributary underflow. Also, accurate estimates of actual evapotranspirative (ET) losses are nearly impossible to make directly from climatologic data so that total precipitation data cannot be directly converted to available recharge.
However, by using the estimate of ground water recharge originating in the southern Bannock Range watersheds, actual ET loss for those watersheds can be determined indirectly. The total area of the Mink Creek, Gibson Jack, and City-Cusick Creek watersheds was assumed responsible for contributing the ground water recharge residual term (>5.5 Bgal/yr, Fig. 1) in the southern aquifer's water balance. This net recharge volume was compared to the total amount of mean annual precipitation available over the southern Bannock Range watersheds in order to estimate actual water lost through evapotranspiration, by difference.
Based on this approach, the amount of ground water recharge originating from these watersheds is almost exactly 20% of the area-weighted available total annual precipitation. Therefore, a fairly confidant estimate can be made of actual ET plus runoff loss in the south Bannock Range watersheds in the 1993/94 study period, wherein ET + runoff comprises 80% of the amount of annual precipitation intercepted. These results were used to estimate available ground water recharge for other watersheds in the LPRV.
An overlay of 30-year mean annual precipitation isopleths (University of Idaho, 1993; Figure 3) was used to estimate available recharge per watershed by weighting the zones within precipitation isopleths for ET + runoff loss. It was assumed that losses varied from 100% of total precipitation in zones with less than 25" of mean annual precipitation to 50% in zones with more than 30" of mean annual precipitation; this proration scheme approximates the 80% overall precipitation loss in southern Bannock Range watersheds as estimated above. The weighting scheme was applied to other watersheds in the LPRV to estimate the amount of water available as ground water recharge to the LPRV aquifer.
The results are summarized in Table 1, showing the net amount of loss-weighted annual precipitation available as ground water recharge in each watershed of the LPRV. From this, the estimated quantity of recharge supplied by the Pocatello Creek drainage to the northern aquifer is of the order of 0.38 Bgal (5.1 x 107 ft3) per year. This estimate is viewed as an upper limit, since the proration of ET + runoff loss was adjusted downward (90% rather than 100%) to derive a non-zero net recharge estimate for this drainage. It appears safe to say that the Pocatello Creek drainage contributes only a small amount of recharge to the LPRV aquifer.
Overall Aquifer Water Balance
With this value as a best available estimate of Pocatello Creek tributary ground water recharge, the water balance for the entire aquifer system can be completed (Figure 4). The pumping withdrawals shown for the northern aquifer represent total 1999 Pocatello and Chubbuck municipal well production combined. The tributary outflow to the Snake Plain is not a direct estimate, and reflects a simple residual calculated from the overall water balance. If the 1999 water balance of the aquifer was similar to the 1993/94 period, then municipal pumping in 1999 represented more than 80% of identified available recharge to the LPRV aquifer system (sum of contributions from the Portneuf Gap, south Bannock Range, and Pocatello Creek).
However, several important caveats should be noted. First, this water balance estimate is considered unusually accurate because of the well-defined geometry of the southern valley; nonetheless, it represents only the relatively shallow, uppermost portion of the sedimentary aquifer system that fills this bedrock graben. We do not know anything about the deeper portions of the system and the possible role that deep fracture systems and fault-controlled permeability may play in routing upper Portneuf basin underflow beneath the currently exploited portion of the LPRV aquifer system.
Secondly, the flux of ground water past Red Hill may not represent all the recharge contributed by the southern valley to the northern aquifer. It is possible that some quantity of recharge orginating in the southern Bannock Range discharges into the northern aquifer directly, bypassing Red Hill (Figure 4, gray arrow). If so, this would represent an additional source of recharge that would add additional aquifer capacity (although no more than ca. 0.5 Bgal/yr or less than 6 % additional capacity).
Finally, the northern aquifer appears to contribute some tributary undeflow to the Snake River Plain (less than 0.5 Bgal/yr). This out-going flux could be considered unused capacity. At current pumping rates, this amount of outflow scales directly with Pocatello Creek's estimated recharge contribution. Should Pocatello Creek actual recharge prove to be less than estimated in Figure 4, outflow to the Snake Plain would also decrease proportinately. If northern aquifer pumping capacity were increased to capture this unused capacity it is possible that the direction of ground water flow in the northern end of the system could be reversed, thereby drawing Snake Plain ground water into the northern LPRV aquifer. Although "overpumping" the northern aquifer may appear to be an attractive option from the standpoint of providing additional water quantity, the risk of pulling in contaminants such as ethylenedibromide or nitrate from the Snake Plain aquifer must be weighed against the increase in system capacity.
Potential Surface Water Supplies
Based on discharge measurements and continuous records made on lower Mink Creek during 1994 and 1995 (S. Van Hoff, unpubl. data, 1995), it appears that surface runoff comprises a minor fraction of the basin's water budget. Baseflow conditions persisted for about 10 months of the year at an average discharge of 2 cfs, with spring runoff accounting for about 12 cfs over about 2 months of the year. This corresponds to less than 3% of the available total precipitation that the Mink Creek watershed receives annually, and to only 13% of the amount of ground water recharge that the Mink Creek watershed contributes to the LPRV aquifer. If half this were diverted to augment water capacity, it would correspond to about 0.33 - 0.4 Bgal annually, or less than a 5% increase over the capacity available from ground water alone. Although this estimate is only approximate, it demonstrates that surface water cannot add significantly to total water capacity.
University of Idaho, 1993, State of Idaho Mean Annual Precipitation, 1961-1990; U. Idaho, State Climate Program, 1:250,000 GIS coverage.
Welhan, J., Meehan, C. and Reid, T., 1996, The Lower Portneuf River Valley Aquifer: A Geologic / Hydrologic Model and Its Implications For Wellhead Protection Strategies; Final Report, EPA Wellhead Protection Demonstration Project and City of Pocatello Aquifer Geologic Characterization Project; 48 pp. plus figures and appendices.