Retention or Detention Pond Earthcache EarthCache

This cache has been archived.

PathfinderMark: The retention pond is gone. Thus the earthcaching lesson. Thanks to all who found it!

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Hidden : 2/3/2010
Difficulty:
2.5 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

As an earthcache, there is no “box” or “container” to discover. Rather, with this cache, you discover something about the geology of the area. For more info, consult www.earthcache.org. The cache is handicap accessible.

The earthcache is located in Oakley, CA. It is a drive-up cache. If the kids are along, they’ll enjoy the lovely and well-maintained park nearby.

Congrads to SnoWake for FTF (First to Finish logging requirements).

Logging Requirements:
Send the answers to #1-#5 to me through my geocaching profile.

1. List the name “GC23G2D Retention/Detention Pond Earthcache” in the first line of your email. Also, list the number of people in your group.
2. Examine some dirt near location. Is the soil COURSE (large grains) or FINE (small/minute grains)? Why?
3. What type of pond (based on the description) is in front of you (Retention—Wet or Detention—Dry)?
4. Is it a “natural” or “unnatural” type of pond? (see description)
5. What is the geologic VALUE of this pond (see description).
6. (Per current gc.com guidelines, photos are no longer allowed to be required. HOWEVER they are encouraged, since they can help clarify that you have visited the location if your other logging requirement answers are vague). Post a picture of yourself and your GPS with your log that shows the fire hydrant and pond fence in the background. DO NOT show any of the pertinent information in your picture or your log may be deleted.

Geology:
The Oakley, CA region is part of the vast sedimentary “Delta” system created by the San Joaquin and Sacramento Rivers as they drain into the ocean. Oakley itself is part of the Central Valley, a very flat valley (averaging just 12 feet above sea level). The flatness of the valley floor contrasts with the rugged hills or gentle mountains that are typical of most of California's terrain. Geologically, the valley is thought to have originated below sea level as an offshore area depressed by subduction of the Farallon Plate into a trench further offshore. The valley was later enclosed by the uplift of the Coast Ranges, with its original outlet into Monterey Bay. Faulting moved the Coast Ranges, and a new outlet developed near what is now San Francisco Bay. Over the millennia, the valley was filled by the sediments of these same ranges, as well as the rising Sierra Nevada to the east; that filling eventually created an extraordinary flatness just barely above sea level; before California's massive flood control and aqueduct system was built (part of the system exists just a few miles north of GZ), the annual snow melt turned much of the valley into an inland lake.

Sediment and Soil Formation:
Soil serves as a natural reservoir of water and nutrients, as a medium for the filtration and breakdown of injurious wastes, and as a participant in the cycling of carbon and other elements through the global ecosystem (Encyclopaedia Britannica Online). It has evolved through the weathering of solid materials such as consolidated rocks, sediments, glacial tills, volcanic ash, and organic matter.

Silt is created by a variety of physical processes capable of splitting the generally sand-sized quartz crystals of primary rocks by exploiting deficiencies in their pysical size (lattice). In this area, that has resulted primarily by weathering of the rock and abrasion of the sand-sized sediments as they were carried (transported) from the mountains to the east. Semi-arid areas such as California create more silt than any other environment! If we were to “dig down” into the silt you can view in front of you it is likely more than 60’ deep! Sometimes, silt is know as “rock flour” and this might be the picture you envision as you examine dirt on the ground in front of you. Over time, silt can be metamorphisized into rock—known as siltstone. This is unlikely to happen in the foreseen future in this area though, since there is little “heat” or “pressure” action to force the silt to compress. (Logging Requirement #2 is based on understanding/interacting with this section)

Retention/Detention Ponds:
As a result of this sediment-centered flatness, rainwater is unable to run-off towards the rivers and oceans without help. In urban areas especially (where pavement adds to the run-off challenge), retention ponds dot the landscape. Traditionally, they have been in Commercial areas, next to large surfaces that have been made impermeable to rainwater (unable to absorb rainwater—such as parking lots and warehouses). However, Residential areas are often overlooked when considering storm water runoff. Many impermeable surfaces such as streets, driveways, sidewalks, walkways, and roofs are in residential subdivisions. Retention ponds capture diverted storm water runoff from these surfaces.

Retainment/Detainment ponds provide two primary services.
1. First, they retain the runoff before releasing it into streams. They release the water at flow rates and frequencies similar to those that existed under natural conditions. The flood volume held in a retaining pond reduces the impact on downstream storm water systems. Residential areas are prone to flooding and must deal with storm water runoff as do large commercial sites such as grocery stores, restaurants, and retail stores. The storm water systems in subdivisions are just as important as commercial areas. The storm water systems flush rainwater quickly from streets and gutters and into the nearest waterway.

2. The second benefit of the retaining ponds is that they provide pollutant removal through settling and biological uptake. Unfortunately, storm water is not treated and often contains many pollutants including car fuel, oil, and sediments. The use of ponds as BMP’s with stormwater systems in urban areas is becoming more widespread. It is already common to see retention ponds in commercial areas. Ponds remove 30-80% of certain pollutants from water before it enters nearby streams. Common pollutants reduced are sediments, bacteria, greases, oils, metals, total suspended solids, phosphorous, nitrogen, and trash (England 2001). Ponds are one of the most effective tools at providing channel protection and pollutant removal in urban streams (www.stormwatercenter.net 2001)

Essentially, retention ponds provide water quality and quantity control.

Two common classifications of retaining ponds are either “wet” or “dry.”

• Wet ponds, known as retention ponds, continually have a pool of water in them called dead storage.
• Dry ponds, known as detention ponds, do not have dead storage and dry out between storms.

Retention ponds are more effective than detention ponds. The permanent pool of water found in the wet ponds is more efficient at removing particle pollutants. It does this by absorbing energy from inflow of the stormwater as it enters the pond, preventing scour material from settling to the bottom, and exchanging new incoming stormwater with previously captured water. This provides extra time between storms for pollution to settle. Aquatic vegetation is often associated with wet ponds. Vegetation such as grasses and plants are able to establish themselves in the permanent pool of wet ponds thus providing extra pollutant removal. The aquatic plants and grasses serve as an extra filter in the pond. They assimilate dissolved pollutants and, by biological uptake, transform pollutants into less toxic materials. Microorganisms often establish themselves in wet ponds and aid in the breakdown of pollutants.

Why are these ponds needed in a residential system?
First, because there are consistently “sediment creation (erosion)” activities going on including: gardening, landscaping, house building, house additions, and of course swimming pool installations, all of which release dirt into the storm water. Second, fertilizing, lawn watering, washing cars, and painting house all have the potential to release toxins and polluntants into the water system. All of these acitivites increase the possibility of sediments, toxins, and pollutants entering the California watersheds and moving downstream to the larger watersystem, in this case the Sacramento Delta system.

There are two types of retention/detention ponds – those that look “natural” (little machinery or tubing visible) and “unnatural” where the storm water pipes and drains are obvious within the embankment of the pond.

Note: This pond filters approximately 5.4 million cubic feet of water (if the pond is full). WOW!

I will only respond if you have incomplete logging requirements. Go ahead and log your cache

Resources:
United States. National Research Council. Washington, DC. "Urban Stormwater Management in the United States." October 15, 2008. pp. 18-20.

Wikipedia “Surface runoff.” (visit link) 2009.

Jordan, Karen J. “The Use of Retention Ponds in Residential Setting.” (visit link) 2002.

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