Contributed Advisory Report
To: U.S. Forest Service (USFS), Arizona Dept. of Transportation (ADOT), Barry Allan, Executive Director, Oak Creek Watershed Council (OCWC)
From: Paul A. Lindberg, AZ Registered Geologist #22226
Date: May 29, 2012
Subject: Pollution Hazard, Redwall Limestone Aquifer; Midgley Bridge Area; Sedona, AZ
PDF file of article HERE
Purpose of this Report:
At the request of Barry Allan, Executive Director of the Oak Creek Watershed Council, I have prepared a brief geological report dealing with hydrological concerns of lower Oak Creek Canyon. It is to be presented to a joint meeting with USFS and ADOT personnel and Barry Allan of OCWC at 9:30 am at the Sedona Ranger Station, May 29, 2012.
The recent discovery of party trash and human feces left behind during the recent Cinco de Mayo holiday weekend along the banks of Oak Creek beneath Midgley Bridge raises serious concerns regarding potential pollution of Sedona’s Redwall Limestone aquifer (Red Rock News, May 4, 2012). Although the Redwall Limestone formation is not exposed at creek level, its apex lies within tens of feet of the canyon floor. While it cannot be directly observed beneath stream boulders, the subsurface Redwall Limestone in the Sedona area is typically riddled with an intricate network of solution cavities, as known from water wells and sinkhole collapse. As such, polluted surface water can readily contaminate the porous limestone strata. This brief report presents the writer’s professional experience in a voluntary report from a concerned citizen.
Geological Setting of Oak Creek Streambed Below Midgley Bridge:
Figure 1 shows a sketch map of the lower section of Oak Creek Canyon extending from the Grasshopper Point area to Uptown Sedona. The trash site below the bridge is shown.
Figure 2 shows a geologic sketch cross section oriented roughly north-south through the bridge area. Midgley Bridge on Highway 89A is located at the base of the overlying, recessive weathering Hermit formation. Immediately below bridge level is the cliff-forming Esplanade
Sandstone member of the Supai formation. Oak Creek has eroded down through this competent sandstone almost to the top of the underlying Redwall Limestone, an important rock formation and principle aquifer for the Sedona water supply.
Figure 3 shows a geologic sketch longitudinal section oriented along the axis of Oak Creek Canyon extending from east of Grasshopper Point to Uptown Sedona. It will be noted that upon entering Uptown Sedona, Oak Creek crosses the Sedona fault at which point the Hermit formation has been dropped down to town level.
ADEQ Flow Measurements of Oak Creek
A couple of decades ago the writer compared notes with ADEQ personnel who were puzzled by a water flow anomaly in Oak Creek at the east edge of town. It seems that during periods of normal low water flow in the creek there is more water entering Uptown Sedona at the outlet of the canyon than the flow measured upstream at Grasshopper Point.
There is a straightforward reason for the increase of water flow entering Uptown Sedona. Redwall Limestone beneath the Sedona town level is riddled with solution cavities that make it an ideal aquifer for the city water supply. Collapse of very large subterranean water-filled caves beneath the greater Sedona area in relatively modern time has created at least seven known sinkholes from the collapse of Redwall solution caves (Lindberg, 2010).
The writer believes that the same set of solution cavities exist in the subsurface limestone lying beneath the bed of Oak Creek below Midgley Bridge. Surface water flow in Oak Creek is relatively constant between Grasshopper Point and Uptown Sedona but there has to be a significant amount of subflow passing through the porous subsurface limestone. At the end of the canyon where Oak Creek crosses the Sedona fault, water is released from subsurface solution cavities where it rises to stream level as artesian water and increases the stream flow (see fig. 3).
Groundwater Contamination Potential:
The writer is concerned that any surface-borne pollution, especially human feces, that is deposited in the lower reach of Oak Creek Canyon poses a serious source of contamination that can leak directly into solution cavities below the base of Oak Creek Canyon, particularly at
places like Grasshopper Point and below Midgley Bridge. Contamination can quickly enter both the surface water of Oak Creek and leak into the subsurface Redwall Limestone aquifer through solution fissures. In doing so, it can create immediate stream pollution as well as long term surface and groundwater contamination downstream.
There is obviously no easy fix for this situation. But the need for some form of policing action is deemed necessary to prevent future abuse of this kind. On-site warning signs need to be explicit and straightforward; Groundwater Contamination Hazard: Do Not Pollute! (or something to that effect)
A sanitary toilet facility below Midgley Bridge obviously brings with it maintenance and policing problems and is probably not be the proper solution. Some form of action, however, is needed to be taken by the combined efforts of the stakeholders that this contributed report is being sent to.
Paul A. Lindberg