Donna F. Nelson and Alan R. Dulaney

A RESOURCE-EFFECTIVE MANAGEMENT TOOL

FOR MULTI-AGENCY PROJECTS THROUGH GIS:

AS IMPLEMENTED IN THE DOWNTOWN TUCSON GROUNDWATER STUDY

INTRODUCTION

Tucson, Arizona, is the largest city in North America totally dependent on groundwater for its water supply. The citizens of Tucson are more concerned than residents of other cities about the quality of their water resource. Ballot propositions about groundwater issues are passionately debated in Tucson, where they might be ignored in other locales. Rapid growth, problems with contaminants, and dropping water tables heighten such concerns.

The purpose of this paper is to present information on a cooperative venture concerning groundwater quality in the area of downtown Tucson. This project is centered about the exchange and management of data among several different parties. Because spatial relationships are important in this area, Geographic Information Systems (GIS) capabilities of the Arizona Department of Environmental Quality will be extensively utilized to bring this project to a successful conclusion. The role of GIS is expected to become major; however, the project is only in its initial stages, and the information presented here today is of a preliminary nature. As time goes on, the data behind these GIS maps will become better and more extensive, filling in the data gaps, and allowing for increasingly sophisticated analytical methods.

THE DOWNTOWN TUCSON AREA

Shallow groundwater beneath downtown Tucson has been contaminated with petroleum hydrocarbons for many years. The petroleum hydrocarbon most commonly detected is diesel fuel. However, there is also evidence of gasoline in the vadose zone and halogenated volatile organic compounds (VOCs) in the soil, in shallow groundwater, and in the regional aquifer.

In the mid 1960s, the City of Tucson drilled two water supply wells, A-033A and A-034A, in the downtown area and found both wells contaminated with diesel fuel. As a result, neither of the two wells were ever used as production well facilities. The City currently does not have any production wells in the downtown area. At least one City well further southeast, Well B-78a, has been adversely impacted by what appears to be diesel contamination, forcing its recent closure.

Newspaper articles appearing in the Tucson Citizen in the late 1960s reported that surface discharges of "diesel oil" occurred in Railroad Wash near 22nd Street. In the mid and late 1980s, results from subsurface investigations indicated that soils were contaminated along the proposed Aviation Parkway corridor and in Railroad Wash adjacent to the Pacific Fruit Express facility. In addition, Total Petroleum Hydrocarbon (TPH) and volatile organic compounds (VOCs) have been detected in shallow groundwater at a number of ADOT monitor wells near the downtown area. These data suggest that petroleum and volatile organic compounds have migrated into the subsurface. Contaminated soils represent a source for continuing contamination of both shallow groundwater and the regional aquifer.

Since the 1960s, regional groundwater levels have dropped below the base of one or more clay/silt layers or lenses beneath the downtown area. The clay/silt sediments appear to impede the vertical movement of contaminants and may promote lateral spreading within the vadose zone. In some areas, shallow groundwater and contaminants may have collected immediately above the impeding zone(s). The lateral extent of shallow perched groundwater and contaminants in that groundwater are not known. Knowledge of the extent of contamination of the deeper regional aquifer is also incomplete, but the recent closure of COT well B-78a suggests that diesel contamination continues to spread.

A wide variety of regulatory programs, potentially responsible parties, community concerns, and possible economic impacts come together in the General Study Area. The General Study Area is in the northwestern part of the Tucson basin in the general vicinity of downtown Tucson. This area includes locales both upgradient and downgradient from downtown Tucson so that the magnitude and extent of the contamination and all the potential sources can be evaluated.

GEOLOGICAL SETTING

The Tucson Basin is surrounded by mountains consisting of Precambrian to Tertiary igneous, metamorphic, and well-indurated sedimentary bedrock. Groundwater is contained in the detrital sediments derived from these mountains. These sediments have been deposited in the subsiding basin since the Tertiary. Basin sediments are divided by Davidson (1973) into three units separated by unconformities, which are (in ascending order): 1) the Pantano Formation of Oligocene age, 2) the Tinaja beds of Miocene to Pliocene Age, and 3) the Fort Lowell Formation of Quaternary Age. The Tinaja beds are further subdivided into three unconformable units, the lower, middle, and upper (Anderson 1988). The upper beds are hundreds of feet thick and composed of gravel, sand, and clayey silt, with coarse-grained sediments towards the edges of the basin and finer-grained sediments common towards the center, which is well to the east of the downtown Tucson area. These upper Tinaja beds also contain a fine-grained facies of calcareous clays which acts as a confining layer wherever it is present, separating an upper and lower aquifer zone at depths ranging from 250 to 350 feet below land surface (Hanson 1988). The overlying Fort Lowell Formation consists of unconsolidated to poorly indurated clayey silts with interbedded sands and gravels (Hanson 1988, Anderson 1988).

Anderson (1988) projects the buried Santa Cruz Fault as crossing the downtown Tucson area, trending NNW/SSE. This normal fault, part of an overall basin graben structure, is postulated to be downthrown to the northeast. It could offset middle Tinaja beds up to 800 feet, but apparently has no expression in the upper Tinaja or Fort Lowell sediments (Anderson 1988). This suggests that no movement has occurred along the Santa Cruz Fault since the Pliocene. Another normal fault extends northeast from the Santa Cruz Fault through the downtown vicinity, with its downthrown side apparently to the southeast. Again only the lower and middle Tinaja beds appear to be offset, not the upper strata. The very uppermost one to two hundred feet of sediments do not seem to be cut by either fault.

REGIONAL AQUIFER

The regional aquifer consists of the Pantano, Tinaja, and Fort Lowell Formations, of which the upper Tinaja beds and Fort Lowell Formation yield the greatest amounts of water (Anderson 1988). The presence of a confining bed of calcareous clays and fine-grained sediments in the upper Tinaja leads to a subdivision of the regional aquifer into a lower and upper aquifer zone across much of the Tucson Basin (Hanson 1988). However, it is not clear that this confining layer extends unbroken under downtown Tucson, where it is more appropriate to speak of the regional undivided aquifer. This regional aquifer has been designated a sole source aquifer by the U.S. Environmental Protection Agency (EPA), and it provides 100% of the public drinking water supply for the Tucson Basin.

The scattered distribution of wells indicates the paucity of knowledge concerning the regional aquifer near downtown Tucson, where it is little utilized. Regional groundwater movement in the downtown Tucson area is generally to the north, although local variations occur. Recharge occurs as a result of underflow into the Tucson Basin from other groundwater basins to the southeast. Infiltration of surface water into the channels of the Santa Cruz River and its tributaries is also a source of recharge for the regional aquifer. Depth to the groundwater of the regional aquifer ranges from 140 to 170 feet across the downtown Tucson area (Haney 1989), becoming more shallow near the Santa Cruz River. Depths to groundwater are deeper towards the center of the Tucson Basin, which lies several miles east of the downtown area. Prior to 1940, inflow approximated outflow in the regional aquifer of the Tucson Basin. But since 1940, overpumping has resulted in water level declines of 100 feet or more in the metropolitan Tucson area (Anderson 1988).

PERCHED GROUNDWATER

Zones of perched groundwater have been noted in several parts of the Tucson Basin (Anderson 1988). An area of perched groundwater was reported in the PAG study (Block et.al. 1989). A large area of perched groundwater has been documented to the south of the downtown Tucson area, extending from the Santa Cruz River west approximately two to four miles (Block and Mertz 1986, Leake and Hanson 1986). The area around the Tucson Airport lies within this perched groundwater zone. Maps of water table contours and depths to water prepared by Tucson Water also illustrate an area of perched groundwater south of downtown Tucson (Babcock, Miley, and Stevens, 1989). Shallow and variable water depths, ranging from 33 to 78 feet, and cascading water in several holes have been reported (Parsons Brinckerhoff et.al. 1986) in the downtown area, indicating the presence of perched groundwater. Cross-sections to the north of the downtown area, however, do not show a perched zone of groundwater. This could be due solely to a paucity of data concerning shallow depths north of downtown Tucson. However, it is also possible that these indications of perched groundwater beneath downtown Tucson represent the northern and northeasternmost extensions of this large area of perched groundwater. North and east of the downtown area the clayey lenses which permit the formation of perched groundwater may be missing, as upper deposits become sandier and coarser.

Depths to perched groundwater ranged from 60 to 85 feet. Leake and Hanson (1987) report perched groundwater thicknesses of one to five feet in the area south of downtown Tucson. Other reports indicate perched groundwater thicknesses of 10-20 feet in the Tucson Basin. This range of thicknesses may hold for the downtown area as well.

The perched groundwater is not currently used for public water supply. Prior to 50 years ago, however, what is now perched groundwater may have been the water table from which Tucson obtained much of its water via shallow drilled and hand-dug wells. It is possible that some domestic wells completed into this now perched zone still exist, although none were found in searches of ADWR well records. As recently as the 1970's, a well was constructed into the perched zone at the El Presidio Garage location as part of the Civil Defense program, indicating that this aquifer was considered suitable as an emergency public water supply. All aquifers in Arizona are protected for drinking water purposes.

While perched groundwater may have originated as remnants of the original water table of the Tucson Basin, it has now developed characteristics such as flow and recharge which are independent of the regional aquifer. Groundwater flow directions within the perched zone are variable, but two major components of flow appear to be to the west and northwest, towards the Santa Cruz River. Perched groundwater is not reported towards the northeastern and southeastern peripheries of the study area. The clayey zones necessary to support any perched layer may not exist to the northeast or southeast. There is no apparent continuity between perched groundwater within the study area and zones of perched groundwater reported to the south, but this may be due primarily to the general lack of data. It is hypothesized that perched groundwater is now associated with the Santa Cruz River and its tributaries across the Tucson Basin. Recharge of the perched zone may be connected with the Arroyo Chico. Aquifer tests have not apparently been performed on the perched groundwater, and thus no estimates of hydraulic conductivity, storativity, or other hydrogeological characteristics are available.

THE DOWNTOWN TUCSON TECHNICAL GROUP

In late 1993 a group composed of technical staff from the Arizona Department of Environmental Quality (ADEQ), Pima County Department of Environmental Quality (PDEQ), City of Tucson, Pima Association of Governments (PAG), and Arizona Department of Transportation (ADOT) began meeting on an informal basis. Discussions within this group provided a basis for plans for further action on the problems of groundwater contamination in the downtown Tucson area. Consensus emerged on several concepts.

The first concept agreed on by all is that the problem must be well defined before appropriate remedial measures can be designed. Much of the contamination appears to be confined to the uppermost shallow perched aquifer anywhere from 60 to 90 feet below ground surface. It is not at all clear, however, exactly how many perched layers exist within this shallow groundwater zone. It is known that at least two such perched layers exist. The degree to which all these potentially contaminated perched layers are in hydraulic communication, or where such transfer of water and contaminants might be taking place, is also unknown. One of the major unknowns is whether or not the shallow groundwater flows in any manner down to the deeper regional aquifer, which is the primary drinking water source for the entire Tucson Basin. Such interaquifer transfers could result in the contamination of the far more important regional aquifer. Questions concerning where such hydraulic communication between the two zones (shallow and regional) might be taking place, and how the transfer occurs, must also be known. Clearly, given the heterogeneity of geological materials in the Tucson Basin, many of these problems are spatial in nature.

The task required then must be a coordinated approach to investigations and cleanup.

It makes very little sense for a party pursuing investigations on one parcel to put in a well into the shallow groundwater zone when a few feet away across a property line the neighboring party has already installed a similar well as part of their investigation. Similarly, if one party begins a remedial project to clean up his property, but causes movement of groundwater and contaminants onto another property, little is achieved in the way of an overall cleanup. The best way to prevent duplication of effort or outright cancellation of cleanup with remedial designs which counteract differing remedial designs is to coordinate all investigations and cleanup projects through one group. The Downtown Tucson Technical Group is the best forum for this coordination of efforts.

Much of the contamination in soils and groundwater in the downtown Tucson are is derived from leaking underground storage tanks (LUSTs). Other sources of contamination include dry cleaning facilities, pipelines, manufacturing sites, and other industrial facilities concentrated within one mile of the major railroad corridor. Many of these industrial facilities no longer exist. It may never be possible to determine how many such historical facilities once existed within a mile of the railroad, a major transportation corridor which itself has existed well over a century, or to determine how many were sources of contamination. All of these properties fall under a variety of regulatory programs, and all have responsibilities for characterizing contamination emanating from or present under their property. Unfortunately, the requirements of each program often differ substantially, and different properties receive varying amounts of regulatory attention depending on which program they are under.

The most important aspect of coordinating efforts amongst all the parties required to pursue investigations and cleanups is to find out what they are all doing. Data-sharing is envisioned as the most important activity of the Downtown Tucson Technical Group. This activity is best managed by the Arizona Department of Environmental Quality, since ADEQ has regulatory authority over virtually all the parties involved.

Another concept accepted by the group is that clean-up goals and remedial technologies will be agreed upon in advance. Free petroleum product floating on the water table in both the shallow and regional groundwater zones must be removed; this is a cleanup goal accepted by all. However, remediation standards for soils are rapidly evolving on both the national and state levels. New Soil Remediation Rules have recently been adopted by the State of Arizona which explicitly allows soil cleanup levels justified by risk assessments. Thus the degree to which soils will be cleaned up will depend largely on the environmental and health risk posed by contaminants in the soil. Such goals are best discussed within the context of the Downtown Tucson Technical Group.

As ADEQ converts to risk-based corrective actions for contaminant problems instead of the old paradigm of fixed numbers for all situations, precise knowledge of receptor locations becomes mandatory. Groundwater receptors are primarily wells. GIS is the most practical means of relating well locations to facilities planning risk-based corrective actions. ADEQ already has locational information on many types of wells (drinking water supply wells, monitor wells, abandoned wells) and is diligently working to get accurate locational data on as many wells as possible. GPS is the preferred means of recording locational data for each well. Many wells in the Tucson Basin have been located using GPS.

It is important to note that this approach to a regional problem is cooperative in nature, not coercive. Although ADEQ is not giving up any of its enforcement powers, in recognition of situations which may require administrative orders and civil action with attendant fines and penalties, it is hoped that the envisioned cooperative, coordinated approach to the problem of widespread groundwater contamination will result in faster, more efficient, and more cost-effective investigations and cleanups for all the participants. In the end, Tucson as a whole is the winner.

GIS AS A FACILITATOR

Although several methods of exchanging data on a regular basis have been suggested, one of the best methods will be the use of ADEQ's Geographic Information System (GIS). ADEQ's

ArcInfo 7.0.3 licenses, a Hewlett Packard 650C plotter, and a Hewlett Packard LaserJet IV printer are being used to store, analyze, and output reports and maps for the Downtown Tucson Technical Group. ADEQ also utilizes modern Global Positioning System (GPS) equipment for acquisition of accurate locational data. ADEQ recently put in place a Locational Data Policy written by the GIS/GPS Steering Committee, which requires ADEQ personnel to obtain sound locational coordinates for facilities and features regulated by ADEQ. Because of the analytical and mapping capabilities available through ArcInfo and the agency committment to data accuracy, ADEQ is well-equipped to perform the function of data management through GIS.

Accurate locational coordinates for most of the LUST sites in the project area were acquired during a special effort in the spring of 1994. Facilities were called prior to fieldwork and permission obtained to enter the property for a few minutes to gather GPS readings. ADEQ personnel visited the known facilities, personally explaining the purpose of the visit and obtaining direct permission from those facilities which could not be reached by phone in advance. Almost all facilities allowed ADEQ personnel entry for GPS data gathering. Types of facilities visited also included state Superfund facilities (including potential source facilities), LUST facilities, and facilities with underground tanks that did not have a release.

GPS activities were directed at gathering data for several types of features deemed critical for the success of the data sharing effort. Wells were of particular interest. All production wells within the project area as well as the surrounding buffer zone were visited in the company of Tucson Water personnel. Monitor wells on known LUST sites as well as Superfund sites were located via GPS technology. Wells represent points of intersection with the shallow and/or regional aquifers, points which yield groundwater quality data, points which must be firmly grounded in locational truth to be used in GIS.

Hydrographic features such as the Santa Cruz River and the Arroyo Chico were also accurately located by GPS. Line files were collected by traversing these surface and semi-surface drainages on foot and by vehicle.

Street grids were developed from census data, TIGR files. These grids will be further refined in the future by judicious collection of accurate GPS coordinates for major intersections. Snapping this grid to several accurate points should considerably reduce any map divergence between street addresses and GPS locations for wells and facilities.

The Arizona State Land Department ALRIS files provided the locations for Interstate 10 and the Southern Pacific railroad lines. ALRIS also provided the township and range overlay for legal descriptions.

The General Study Area was defined as an area one mile out from the Southern Pacific main line, due to the historical association of industrial sites with this transportation corridor. The area is bounded to the northwest by Interstate 10 and the Santa Cruz River, and runs to the southeast to just past the 22nd Street railyard. The buffer zone extending beyond the project area puts that area in the context of the remainder of central Tucson.

Points on the maps are tied through Info to ADEQ's Groundwater Quality Database, which has been developed over the last seven years to include a considerable body of water quality analyses.

MAPS AS MANAGEMENT TOOLS

To date several base maps have been prepared for the General Study Area, which extends out one mile either side of the main railroad corridor from roughly the Santa Cruz River to just past the 22nd Street yard. This area encompasses virtually all of the industrial activity in the Tucson area over the last century which may have released contamination into the subsurface. Wells are also shown on this map.

Four of the maps indicate wells with groundwater analyses which have shown one of four major contaminants to be present. These four major contaminants are tetrachloroethylene (also known commonly as PCE), a related compound trichloroethylene (TCE), a gasoline additive 1,2 dichloroethane, and benzene. The first two are industrial solvents and known carcinogens, and are usually associated with Superfund sites. The latter two are associated with gasoline, and thus with LUST sites. Wells where contaminants were found in levels that exceed maximum contaminant levels (MCLs) set by the U.S. Environmental Protection Agency are indicated by color coding as indicated in each maps legend. Such excedances of MCLs are a cause for regulatory action because they represent health threats that the general public cannot avoid when they drink the contaminated water. Many of the excedances are clearly associated with facilities where, again, extensive investigations have been performed. Investigations in areas lacking good data might turn up more such MCL excedances.

One map indicates wells into the regional and perched aquifers and those which have been found to have free petroleum product. Diesel contains only minute amounts of carcinogenic compounds that could dissolve into groundwater, and thus poses little health risk. Nonetheless, the general public has long been aware of diesel free product on the groundwater table and finds the idea of diesel in water noxious and objectionable. This map demonstrates the association of free product on groundwater with LUST sites. Current remedial activities involve removal of free petroleum product from several wells completed into the uppermost aquifer.

CONCLUSIONS

Covers illustrate the project area, wells installed into both the shallow and regional aquifers, the known extent of VOC contamination in both aquifers, and different types of facilities within the project area. Because the various plumes are still not fully characterized, areas of critical data gaps where more investigation is needed are also pictured.

Because the flow of information is so important to the success of this project, Arc-Info is envisioned as a major management tool to foster cooperation in the investigation and cleanup of this large plume. Without Arc-Info it is doubtful that any significant coordination between investigators could be achieved, and remediation costs would become greater.

REFERENCES

Anderson, S.R., 1988. Potential for aquifer compaction, land subsidence, and earth fissures in the Tucson Basin, Pima County, Arizona. U.S. Geological Survey Hydrologic Investigations Atlas HA-713.

Babcock, J.A., Miley, T., and Stevens, C.R., 1989. Annual Static Water Level Basic Data Report Tucson Basin and Avra Valley Pima County Arizona 1988. City of Tucson, Tucson Water, Planning and Technical Services Division.

Block, M.W., Kushner, G.F., Grimaldi, R.R., Burchard, G.C., and Wieland, D.L., 1989. Metropolitan Tucson Basin water quality and pollution source assessment. Pima Association of Governments.

Block, M.W., and Mertz, A., 1986. Santa Cruz River Alignment Recharge Study. Pima Association of Governments.

Davidson, E.S., 1973. Geohydrology and water resources of the Tucson Basin, Arizona. U.S. Geological Survey Water-Supply Paper 1939-E.

Hanson, R.T., 1988. Aquifer-system compaction, Tucson Basin and Avra Valley, Arizona. U.S. Geological Survey Water-Resources Investigations Report 88-4172.

Hensley, J.R., 1985. Volatile organic compounds in drinking water wells, Pima County, Arizona: an interim report. Pima County Health Department.

Leake, S.A., and Hanson, R.T., 1986. Distribution and movement of trichloroethylene in groundwater in the Tucson area, Arizona. U.S. Geological Survey Water-Resources Investigations Report 86-4313.

Parsons, Brinkerhoff, Quade, and Douglas, Inc, 1986. Aviation Corridor Highway, (State Route 210), junction I-120 to junction SR-810: Subsurface investigations, Phase I, geotechnical report. Submitted to Arizona Department of Transportation, Highways Division.

Donna F. Nelson, Senior GIS Analyst

Alan R. Dulaney, Hydrologist III

Arizona Department of Environmental Quality

Office of Waste Programs, Underground Storage Tank Section

3003 N. Central Avenue

Phoenix, AZ 85012

Telephone: (602) 207-4309

Fax: (602) 207-4346