The Urbanized Effect on Detention Ponds

Travis Jones, Department of Earth Sciences, University of South Alabama, Mobile AL, 36688. E-MAIL: tj_mht@yahoo.com.

            Many innovative ideas have been proposed and implemented to correct excess stormwater in ever-growing urbanized areas. Fairly simple ways to alleviate stormwater are the development of retention ponds. These detention basins create a permanent pool throughout the year that helps remove sediment and pollutants from excess runoff before entering a stream and control the outflow of water into streams after storms helping to eliminate flooding and regulate runoff. This provides a unique way of not only controlling stormwater through watersheds but also provides an ecologically friendly alternative to high maintenance canals. However, some ponds were not even suggested to be used as detention ponds but were designated over time. This study measures significant water quality difference in three different bodies of water now being used as detention ponds. Three detention basins were tested weekly for pH, hardness, alkalinity, turbidity, and dissolved oxygen saturation. The results show slight differences in water quality between detention basins located in older neighborhoods and ones in recently developed or undeveloped areas. While some tested areas differed, all the detention ponds remain at adequate and healthy levels for aquatic life. The development and design of lakes and ponds with the intent of being used for Detention provide an efficient means to aid in runoff and flooding.

            Keywords: detention ponds, urbanization

Introduction

Throughout Mobile, urbanized areas are continuing to develop at an alarming rate. Newer subdivisions are growing with an increased interest in stormwater runoff. Mobile County has declared that the rate of release across the boundary of a site can be no greater after development than before (Department of Urban Development 1998-2002). This poses a problem for developers, as most instances of development will, in effect, increase runoff. A common strategy used for stormwater management is to store excess water on or near the site in a detention basin and then to release it over time (Marsh 2005). These detention basins create a permanent pool throughout the year that helps remove sediment and pollutants from excess runoff before it enters a stream and control the outflow of water into streams after storms helping to eliminate flooding and regulate runoff (Mays 2001).

Detention ponds are one of two types of stormwater ponds commonly constructed. Retention ponds are dry and fill rapidly during a rainfall event gradually releasing water into the watershed until the basin is dry again. Detention ponds are permanent pools and gradually release stormwater through an outlet structure to adjacent surface waters rather than through infiltration into the soils. Detention ponds can be designed as wet or dry. Wet detention ponds are constructed so that the pond bottom is below the seasonal high water table (SHWT) elevation. Dry detention ponds set the pond bottom above the SHWT (Dykehouse 2001).

Although, in theory, detention ponds create an adequate way to balance urban impacts on watersheds, the design must be appropriate to the situation. Poor design will significantly increase a broad range of impacts. These impacts include a loss of habitat by suspended silt smothering organisms, minimal levels of dissolved oxygen, introduction of pathogens such as viruses and bacteria, excessive nutrient levels that can cause an overgrowth of algae, increased temperatures that affect aquatic life, and a degraded aesthetic quality due to litter (Water Environment Federation 1992).

            Many new design elements have been proposed and most developers are concerned with the maintenance of their detention ponds. Karen Jordan’s research in 2001 suggests that detention ponds benefit the Dog River watershed because they control sedimentation flow. Basins like the one found near Charleston Pointe have provided better water quality downstream. It is suggested that the design of these ponds be incorporated into subdivisions when needed.

There are three lakes that I specifically looked at due to their location, their origin, and their surrounding area (Figure 1). The three lakes include Lake McLean, located just off of Demotroplis road at the conjuction of the Spencer and Moore branch. The second body of water tested is Pine Lake, located near Knollwood Hospital on Girby Road. The third area is a newly constructed pond in the Wynnefield subdivision off of Sollie Road on Milkhouse Creek. While used as detention ponds now, each lake’s use in the beginning was designated for different purposes.

McLean Lake was developed in the early 1980s and a bulkhead was created at the southern end of the lake to support the channelized Spencer Branch without the lake being drained.  The dam separating the lake from the channel is very low to allow overflow from the lake during a flood to easily access the canal. However, Jill Daniels, a long time resident on the lake, stated that overfill of the lake is rare. For example, stormwater from Hurricane Katrina only raised the lake a foot by Jill’s recount. This could be due to high soil infiltration producing underground access to the Spencer Branch canal system.

            The area around Pine Lake serves as a great example of a watershed in its undisturbed state. The surrounding area is now home to the Environmental Studies Center, a natural sciences education facility designed to provide unique learning experiences not typically available in the local school. Lloyd Scott, director of the center, noted that the surrounding wetland has always been there and only recently has its use as detention pond been examined. He states that the growing development in nearby neighborhoods, specifically north of the lake has provided more drainage water than in the past. The lake provides a unique look into what type of water is entering the lake from urbanization.

            The Wynnfield subdivision is a new housing project located off of Sollie Road. The design of Wynnfield subdivision detention pond is unique in that resembles ponds found in natural wetlands. The pond is stretched out like a canal and acts like an extended detention pond keeping water in for longer periods of time. Longer detention times have been found to provide optimal pollutant removal (Whipple 1983). These ponds are usually designed for the purposes of providing water quality treatment for the first flush of stormwater runoff, and may also provide quantity control for small storm events necessary to minimize downstream bank erosion. Banks along this pond were not steep but the houses are at least 25 feet from the water, as regulated by housing codes. Water testing in this pond is useful in providing clues into the early stages of detention ponds compared to the older lakes. While all three lakes have had different intentions, they all have become important bodies of water during periods of stormwater runoff. However, the health of these lakes could be affected by the surrounding neighborhoods.

Research Question

            Is there a significant measurable difference in turbidity, pH, hardness, and dissolved oxygen in the detention ponds found adjacent to older subdivisions, newer subdivisions, and undeveloped areas in the Dog River watershed (Figure 2)?

Methods

Each lake was tested for pH, turbidity, alkalinity, hardness, and dissolved oxygen content to determine whether these elements are high or low compared to the other ponds of different neighborhood concentrations. Water testing took place every Tuesday between the hours of 10am and 12pm so to collect samples during the lake’s highest point of dissolved oxygen content. Areas were also monitored during significant rain events which did not occur until the end of testing period. Samples were tested using a LaMotte water quality testing kit used by Alabama Water Watch. Results were entered into Excel.

Results

 The pH of water determines the solubility or the amount that can be dissolved in the water and biological availability of nutrients (such as phosphorus, nitrogen, and carbon) and heavy metals (such as lead, copper, cadmium, etc.). No significant differences were found in each lake's pH. All lake’s pH average is 6.5 SIU with the pond at Wynnefield receiving a slightly higher pH than the others. This pH level is in the optimal range for aquatic life.

 Turbid water, appearing cloudy or muddy, is caused by sediment, algae, and other particles suspended in the water. Stormwater runoff carries soil and debris into detention ponds from the surrounding landscape. Erosion of the pond’s shoreline also contributes to turbidity. Bottom-feeding fish can cause a lot of turbidity as they stir up the bottom sediments in search of food. Rooted aquatic plants have a hard time growing in turbid water and without such plants covering the pond bottom, sediments are more easily resuspended by wind and waves (Water Environment Federation 1992). Turbidity was apparently different in Lake McLean compared to the others (Figure 3). Lake McLean showed turbidity results averaging 23 JTU while the other two lakes were much lower at 5 JTU. A jump in turbidity at the end of the testing period is due to resuspension of sediment during a rainfall event. The cloudiness in Lake McLean could be due to a rather large algae bloom in the northern portion of the lake producing large amounts of oxygen.

Hardness is a measurement of the concentration of metal ions such as calcium, magnesium, iron, zinc etc, usually acquired as rainwater percolates through rock. In most water it consist mainly of calcium and magnesium salts, with traces of other metals(Water Environment Federation 1992). The hardness for each lake was slightly different. On March 20, 2006, all lakes had a hardness count of 30 mg/L. In time, each lake’s hardness fluctuated differently. The pond at Wynnfield had a fluctuation range of 20 mg/L revealing different amounts of limestone at the time. The hardness at Lake McLean slightly dropped by 5 mg/L while the opposite occurred at Pine Lake. All results were indications of moderately soft water.

Alkalinity refers to the hardness derived mainly from carbonate and bicarbonate ions and directly reflects the buffering capacity of the water. The alkalinity showed similarities to each lake’s hardness count. Lake McLean’s average alkalinity was at 25 mg/L with a range of 5 mg/L. The other two lakes averaged between 30 and 35 mg/L with Pine Lake slightly dropping by the end of the testing period and Wynnefield increasing slightly. All results are considered normal values for the area and provide a stable environment for aquatic life.

              The final area tested was dissolved oxygen. Oxygen is essential for aquatic animals and plants. As water moves past their gills, microscopic bubbles of oxygen gas in the water, called dissolved oxygen, are transferred from the water to their blood. Like any other gas diffusion process, the transfer is efficient only above certain concentrations. In other words, oxygen can be present in the water, but at too low a concentration to sustain aquatic life. Oxygen also is needed by virtually all algae and for many chemical reactions that are important to lake functioning. The results were the most significant as seen in Figure 4.  Each lake had its own distinct DO saturation level (Figure 4). Water temperatures in all lakes increased and decreased at the same time during the period. The long period of hot, dry weather gradually increase all ponds’ water temperature during the testing period. Water temperature directly affects DO saturations in that warmer water tends to hold less dissolved oxygen. Lake McLean had a high DO Saturation percentage that is considered too high for many fish with an average DO Saturation of 134%. This could also be due to abundance of algae in the lake. Pine Lake fell into the acceptable range for most animals with an average result of 75%. However, this lake is much larger than the others and may have slower time producing oxygen for such a volume of water. Surprisingly, the newest of the ponds tested showed the best percentages. The newly constructed pond in the Wynnefield subdivision had average values of 110% and fell into a range excellent for most aquatic life.

Discussions and Conclusion

It appears that the detention ponds in the watershed are able to maintain a healthy life before, during, and after urbanization. The new detention pond at Wynnefield appears to be the healthiest. This is mostly due the new building codes and mindful contractors working together to produce healthy ponds that not only control stormwater and pollution but also create new and long lasting aquatic life along the watershed. However, maintenance is crucial for the ponds to remain at an excellent range of health. This is evident when compared to Lake McLean and the amounts of algae causing drastic results in DO saturations. Water quality trends of both the canal at Wynnefield and Pine Lake should be kept to monitor any abrupt changes during the ongoing periods of housing development.

            Overall, detention ponds appear to have a significant impact when regulated and designed properly. New building codes control flooding of homes while design and outlets provide better coverage and access to recycling water. Other ponds and lakes naturally established should not be altered but should be able to access outlets to the watershed, like Pine Lake.

References Cited

Dykehouse, Terry. “Retention Ponds and Detention Ponds, The Recovery Process”. James Edmunds & Associates, Gainesville FL. Available: http://www.florida-stormwater.org/pdfs/pondsarticle.pdf Accessed: March 3, 2006.

Jordan, Karen. “The Use of Retention Ponds in Residential Settings”. University of South Alabama Department of Earth Sciences, Mobile, AL, 2001.

Marsh, William. Landscape Planning: Environmental Applications. Hoboken: John Wiley & Sons, Inc, 2005.

Mays, Larry. Stormwater Collection Systems Design Handbook. New York: McGraw-Hill, 2001.

Subdivision Regulations for the City of Mobile, Alabama. Department of Urban Development. City of Mobile, Alabama, 1998-2002.

Water Environment Federation. Design and Construction of Urban Stormwater Management Systems. Alexandria, Virginia, 1992.

Whipple, William, et al. Stormwater Management in Urbanizing Areas. New Jersey:Prentice-Hall, Inc, 1983.