Groundwater Recharge from a Portion of the Santa Catalina Mountains

Transcription

1 Groundwater Recharge from a Portion of the Santa Catalina Mountains Item type text; Proceedings Authors Belan, R. A.; Matlock, W. G. Publisher Journal Rights Arizona-Nevada Academy of Science Hydrology and Water Resources in Arizona and the Southwest Copyright, where appropriate, is held by the author. Downloaded 16-Jul :47:44 Link to item

2 GROUNDWATER RECHARGE FROM A PORTION OF THE SANTA CATALINA MOUNTAINS by R. A. Belan and W. G. Matlock INTRODUCTION Tucson, Arizona's only source of water is from the underlying groundwater aquifer. For effective groundwater resource management all discharges and recharge sources of the aquifer must be known. Unfortunately, all have not been quantified; of particular interest is the mountain front recharge to the Tucson Basin. Very little research has been conducted to define the recharge of the various foothill regions surrounding the basin. The basic objective of this study was to analyze the geohydrology of a portion of the Santa Catalina Mountains including definition of aquifer systems and their continuity throughout the foothills and to calculate groundwater recharge to the Tucson Basin from the foothills area. Figure 1 is a location map of the study area. GENERAL DESCRIPTION OF AREA PRECIPITATION. RUNOFF AND RECHARGE Precipitation occurs in two seasons. Summer rainfall is characterized by high intensity, small areal extent and short duration, while the winter precipitation is of greater areal extent and longer duration. The annual precipitation in Tucson is about 11 inches, increasing with elevation to about 30 inches in the highest portion of the Santa Catalina Mountains (Sellers, 1960). Much of the precipitation runs off the land surface because of shallow caliche zones that impede infiltration (Abuajamieh, 1966) or evaporates from the soil zone. The only significant areas of recharge are the stream channels and larger washes in the basin, and possibly faults and joints of the mountains and foothills. The greatest The authors are former Graduate Research Assistant and Agricultural Engineer, respectively, Soils, Water and Engineering Department, The University of Arizona, Tucson. Approved for publication as Journal Paper No Arizona Agricultural Experiment Station. 33

3 : N. j.i rii/ 1: R l \ i.. i ' 1 R I3 E R14 E --- S.. i. :. : ARIZ NA Figure I - Location map of study area (land grid) and drainage.

4 individual drainage areas in the study are Finger Rock and Pima Canyons. Summer storms tend to produce sudden, often violent flows that may be heavily laden with silt which retards infiltration directly into the ground or in stream channels (Schwalen and Shaw, 1957). Winter precipitation often produces runoff which readily infiltrates in stream channels and recharges the groundwater system. GEOLOGY The Tucson Basin is surrounded by mountain ranges and was formed in the mid -Tertiary period (Maddox, 1960, Pashley, 1966). The Basin has since been filled with alluvium from the mountains which are composed chiefly of metamorphic gneisses and schists (Maddox, 1960, Medhi, 1964, Abuajamieh, 1966, and Pashley, 1966). The study area is divided into four formations. First is the Santa Catalina gneiss, which forms the lowest boundary, and is composed of highly fractured, metamorphic granites ( Medhi, 1964, Abuajamieh, 1966, and Pashley, 1966). The Pantano Formation is the next oldest and was described by Brennan (1957), Streitz (1962) and Medhi (1964) as generally consolidated dark red or red -brown to purplish conglomerates and mudstones with some gypsum. Voelger (1953) and Pashley (1966) called this the Rillito Formation. A "basin fill" unconformably overlying the Pantano consists of fine sands and silts or coarse sands and gravels weakly cemented (Pashley, 1966). This seems to correspond to the "deformed gravels" of Abuajamieh (1966) and forms the alluvial fans of the foothills. Last is the Recent alluvium that fills the present wash and stream channels and flood plains (Schwalen and Shaw, 1957 and Pashley, 1966). This formation consists of unconsolidated silts, sands and gravels increasing in size towards the mountains to coarse sands, gravels and boulders. The eastern half of the foothills is deformed by faults and joints. The Largest fault is the Santa Catalina Fault, running east to west at the base of the mountains. It was accurately traced in the eastern half of the study area ( Medhi, 1964, and Pashley, 1966), but in the western half the fault disappears under the alluvial fan and can only be inferred. PROJECT DESCRIPTION WELL NET A well network was established in the study area by consulting well 34

5 logs and drillers records in the files at The University of Arizona, United States Geological Survey and local well drillers' offices. The wells were drilled primarily for domestic supplies. There were two main periods of well construction in the study area. The first was around 1930 when most of the wells along the northern portion were drilled or dug. The second period began in 1947 and continues to the present day. Wells drilled during the later period are located mostly along Rillito Creek and Oracle Road. Some wells were abandoned and filled with rubble. In that case the location, bench mark elevation, and any recorded or estimated water levels were still useable. All other wells were measured in March, The base year 1930 was used for detailed study of the area as the water level information was more complete than that of recent years, and the water level was relatively undisturbed by pumping at that time. WELL LOGS AND GEOLOGIC PROFILES Geologic profiles based on well logs were made to correlate geohydrologic conditions and water level information. Well logs were dated from 1929 to 1971 with the older logs generally providing more descriptive geologic information. The well logs that contained the most details were used. In particular the interest was centered on water bearing horizon s. The well logs and resulting profiles showed a high variability of subsurface geology and attemps to make stratigraphic correlations between wells were unsuccessful. Well depths range from tens of feet to over 500 feet. The shallowest wells are along Rillito Creek, and wells increase in depth going northward to the Santa Catalina Mountains. Apparent artesian conditions were found throughout the foothills with water table aquifers along Rillito Creek and a few areas next to the mountains. The cyrrent depth to water in the foothills is from 100 feet to 380 feet, and also tends to increase toward the mountains. WATER LEVEL CONTOUR MAP A water level contour map for 1930 shows the pattern of groundwater flow in the foothills. (Figure 2). The contours shown are the result of analysis of data from available wells, field observations, and study of the various geologic profiles. Contour lines are dashed in areas of uncertainty. Interpretation in the eastern half of the area is complicated by faults and joints. The groundwater gradient varies from about 400 feet per mile in the northwest to about 60 feet per mile in the southeast. Contour lines along Rillito Creek were smoothed to reduce irregularities caused by local pumping and recharge from the flow in the creek. Comparing the 1930 and 1972 water level data showed that very little change has occurred in the water levels in the foothills themselves, but some 35

6 lowering has occured along Rillito Creek because of heavier aquifer development. DISCUSSION OF GEOHYDROLOGY AQUIFERS The well logs and water level information showed small sand and gravel aquifers primarily along the washes of the mountain front area which are perched relative to the Tucson Basin and local in nature. The only aquifers continuously traceable northward from the Tucson Basin aquifer were found along Pima and Finger Rock Canyon Washes in Section 12 of T13S R13E and in Section 22 and 23 of T13S R14E. These two aquifers which form the largest developed aquifers in the foothills were identified in troughs in the surface of consolidated sediments described by Maddox (1960). Groundwater flow in the area between the defined aquifers is extremely small or nonexistent. This observation is consistent with the "dry" wells scattered throughout the area. RECHARGE TO FOOTHILLS AQUIFERS Water in the study area occurs as surface runoff, groundwater under - flow and water which is, for the most part, locked in very low permeability materials. The surface runoff which occurs during times of rainfall and snowmelt gathers into the washes and drains into the lower basin area along Rillito Creek. Little of the surface runoff is thought to recharge local aquifers because of the tight materials under the Recent alluvium and limited duration of the flows. Recharge does occur in some sections of the washes and close to the mountains where the washes and faults cross or coincide. Significant recharge to the various sand and gravel aquifers also occurs directly through faults and joints in the mountains and higher foothills. FLOW-NST ANALYSIS OF RECHARGE TO TUCSON BASIN The study area was divided into 15 flow channels of which eight were considered to contribute significant groundwater recharge to the Tucson Basin (Figure 3). The Darcy equation was used in calculating the mountain front recharge with the total discharge computed as the sum of the discharges of the eight flow channels. The widths and hydraulic gradients of the flow channels were found by flow net analysis of the 1930 water level contour map, by analysis of well logs and geologic profiles and by field observations. Transmissibility (T) values were computed from specific capacities (Cs) of wells representative of each channel based on the method of Thomason, et al, (1960): T = Cs x

7 AR13E R14E T 12 S T13S s_. A a.,l-.,s `-". 00 () O ` i- r. \IA j p 32:1 37 () \ `. ' bo 3,6ó0 o o N a () () O. MEASURED WELLS ()DRY HOLES OTHER WELLS a ey Figure water level contour map.,zoo A Co i O.-`,WATER LEVEL CONTOURS

8 All specific capacity values were determined from well log information except for two flow channels. Specific capacities for these channels were obtained from short term pumping tests. The groundwater underflow across the 2400 foot water level elevation was then calculated for each lettered flow channel (Table 1). The areas between flow channels were considered to have near -zero transmissibility values, thus no significant flow in them. The resulting individual flows range from 4800 gallons per day (gpd)iu.channel G to 180,000 gpd in channel B. The total recharge was 300,000 gpd (336 acre -feet per year) or about 50 acre -feet per year per mile of mountain front as compared to the 325 acre -feet per mile per year obtained in an electric analog model study (Anderson, 1969). Total recharge from the study area represents less than one percent of the annual recharge to the Tucson Basin. CORRELATION OF WATER QUALITY AND GROUNDWATER FLOW A water sample was taken where possible from each well in the study area for chemical analysis including total dissolved solids, ph, hardness and common constituents found in local groundwaters such as sodium, calcium, chloride, sulfate, magnesium and nitrate. Good correlations of water quality and groundwater flow were found for the flow channels of Figure 3. That is, water quality was uniform in a single flow channel but different from other channels. Anomalous qualities were explained by geologic variability, e.g., contact with the Pantano Formation, faults or mixing of groundwater flow. Excellent correlation was obtained in the far eastern portion of the study area in flow channels G. and H. CORRELATION OF WATER TEMPERATURE AND GROUNDWATER FLOW Groundwater temperatures ranged from 66.0 F to 78.8 F. The temperature readings were not in situ measurements, and were used to show general trends with fairly good results. Differences of about 5 F or more were considered significant. Water temperatures show the influence of recharge sources such as from snowmelt from higher elevations to the north. The generally lower temperature of groundwater in wells along Rillito Creek also is indicative of the snowmelt recharge from the Creek. Higher water temperatures found in two wells near Rillito Creek indicate minimal influence from Rillito Creek recharge, probably because of separation by low permeability materials. Water from wells in flow channels B and C is warmer than that along Rillito Creek. There is a slight tendency toward increasing temperature away from the mountains in these channels. This may be because of longer contact with a warmer formation in the subsurface and possibly heat contribution from fault areas. 37

9 T12 T13 B ` '''ylzifo R13.E R 14 E Figure 3- Recharge flow channels. v C7 Ñ r ' WA LEVEL CO. TER NTOUR

10 Table 1. RECHARGE TO TUCSON BASIN Flow Channel Specific Trans- Gradient Width Capacity miscibility (ft /mi) (mi) (gpm /ft) (gpd /f t) Discharge (gpd) A ,600 B ,000 C ,000 D ,000 E ,000 F ,000 G ,800 H ,000 TOTAL (ROUNDED) 300,000 or 336 AF /year 3ß

11 Water temperatures in flow channels G and H correlated well and confirmed other evidence of this groundwater flow area. Mixing with Rillito Creek groundwater occurs near the Creek. CONCLUSIONS The study area is an extremely complex transition zone between the mountains and the lower groundwater basin, and is representative of the foothills surrounding Tucson. Several aquifers showing unconfirmed signs of artesian pressure were located with the two largest developed aquifers in Section 12 of T13S R13E and in Sections 22 and 23 of T13S R14E. Water table aquifers are found near the mountains. All of the aquifers seem to be comprised of sediments that have filled old channels in the semi - consolidated Pantano formation which explains, in part, how extreme variations in quantity and quality of groundwater may occur in relatively short distances. Groundwater gradients range from about 400 feet per mile in the northwest to about 60 feet per mile in the southeast. The depth to water varies from 100 feet to 380 feet. Calculation of recharge to the Tucson Basin from the foothills study area by the Darcy equation is subject to considerable error. Groundwater gradients taken from the water level contour map were reasonably accurate but flow channel width and transmissibility values were subject to question. The use of specific capacity data to calculate transmissibility involves assumptions`of aquifer uniformity obviously not met. Specific capacity is, itself, dependent on well construction and development and the effects of time on casing perforations. Flow channel widths were estimated from geologic profiles, flow net analysis of the water level contour map, and field observations. Flow net analysis used to quantify groundwater recharge was limited by the scarcity of information and the complexity of the area. The total recharge was calculated to be 336 acre -feet per year. This represents about 50 acre -feet per mile of mountain front per year. The study was limited by the low density of the wells or complete lack of wells in certain sections. Assuming transmissibility values of zero for inter -aquifer areas depicts conditions to the best of present knowledge. The true recharge is thought to be within the range of +100 percent to -50 percent of the value given but may be different by an order of magnitude. 39

12 REFERENCES CITED Abuajamieh, M. M., The Structure of the Pantano Beds in the North - ern Tucson Basin: Unpublished M. S. Thesis, The University of Arizona, Tucson. Anderson, T. W., Electric - Analog Analysis of the Hydrologic System, Tucson Basin, Arizona: City of Tucson, U. S. Bureau of Reclamation, The University of Arizona and U. S. Geological Survey, Open File Report, Tucson, Arizona. Brennan, D. J., County, Arizona: Arizona, Tucson. Geological Reconnaissance of Cienega Gap, Pima Unpublished M. S. Thesis, The University of Maddox, G. E., Subsurface Geology Along Northwest Rillito Creek: Unpublished M. S. Thesis, The University of Arizona, Tucson. Medhi, P. K., A Geologic Study of the Pontatoc Mine Area: Unpublished M. S. Thesis, The University of Arizona, Tucson. Pashley, E. F., Structure and Stratigraphy of the Central Northern and Eastern Parts of the Tucson Basin, Arizona: Unpublished Ph.D. Dissertation, The University of Arizona, Tucson. Schwalen, H. C., and R. J. Shaw, Groundwater Supplies of Santa Cruz Valley of Southern Arizona Between Rillito Station and the International Boundary: Agricultural Experiment Station, The University of Arizona, Tucson. Sellers, W. D., 'The Climate of Arizona," in Arizona Climate, The Institute of Atmospheric Physics. The University of Arizona, Tucson. Streitz, R. K., Subsurface Stratigraphy and Hydrology of the Rillito Creek -Tanque Verde Wash Area, Tucson, Arizona: Unpublished M. S. Thesis, The University of Arizona, Tucson. Thomasson, H. G., F. H. Olmsted, and E. F. LeRoux, Geology, later Resources and Usable Groundwater Storage Capacity of Part of Solano, California; U. S. Geological Survey Water Supply Paper Voelger, K., Cenozoic Deposits in the Santa Catalina Mountains near Tucson, Arizona: Unpublished M. S. Thesis, The University of Arizona, Tucson. 40