When we think of dredging we automatically envision a large machine removing copious amounts of sediment from rivers or harbors either through suction or with a grabber. These types of dredging techniques have been the main solution for dying ponds, lakes, retention basins, and rivers. Dredging in this manner physically removes sediment, some excess nutrients, pollutants and organic matter. By increasing the depth of water, weed growth is discouraged, water temperature lowered and oxygen levels increased. However, other issues can occur, for example:
– Disrupts the aquatic ecosystem through physical removal of both plants and animals.
– Bathymetry can be altered, potentially eliminating shallow water habitat essential to spawning, nesting and rearing of
certain aquatic species.
– Causes silting and increases in turbidity, which stresses and can kill aquatic life
– Mounds of sludge can occur on shore
– Increases odors
– Exposes biota to heavy metals and other toxins
– Increases costs if dredged materials need to be transported and relocated to landfill or another site
– Extra costs can occur if material is classified as hazardous waste
– Dredging activities may be subject to regulation under the Clean Water Act.
There is an alternative. Biological Dredging.
Biological dredging is a gentle yet powerful tool to treat a disturbed ecosystem plagued with pollutants and build up of organic sludge at the bottom of the water column. This provides a potential alternative to mechanical or hydraulic dredging.
Beneficial microorganisms have an appetite for a surprising number of compounds, with an ability to turn complex substances, no longer usable to other life forms, back into simple, usable compounds. They are able to degrade sludge and pollutants dissolved in the water column, or attached to sediment, turning them into oxygen, carbon dioxide, water, and more bacteria. The microorganisms themselves will die back to a normal population once their food source is diminished.
A site may require a larger bacterial population to be maintained to deal with the consistent input of pollutants associated with the sediment. The long-term effect of bacterial use in the maintenance of a water body is the eventual reduction in the amount of sediment mechanically dredged out of the lake, as the organic compounds (the sludge) will have been removed. This in turn will reduce the amount of time spent dredging, the amount of sediment dredged, and therefore, its associated cost.
– Uses beneficial microorganisms to degrade in situ (on site)
– Degrades a wide variety of contaminants in sediment and water column
– Less stress on fish and the ecosystem
– No mounds of sludge, in fact there should be a decrease in sludge material
– No odors
– Natural bathymetry is kept intact
– Minimal disturbance to benthic (bottom dwelling) plants and animals
– As sludge diminishes less overgrowth of plant life
– The microorganisms die back to a normal population once their food source is reduced
– If mechanical dredging is needed, there is less sediment and therefore, costs are reduced.
There are a few limitations to this approach:
– Beneficial microorganisms cannot degrade dirt, rock, sand or other inorganic substances
– Works slower in fluctuating extremes of temperature and pH, but they still degrade
– Takes a minimum of one to two seasons to show sustainable verifiable results
– When high levels of metals or biocides are present, formulations may need to be adjusted.
A Biodredging Case Study: Bellvale Lake 2018
Bellvale Lake is a shallow manmade 17 acre lake that feeds a protected stream which empties in to a local reservoir. Three sides of the lake are wooded, the fourth side is the dam which is a paved road with grass and shrub edge. The Lake seems to date from 1959 and was left unattended for the first 40 years. The purpose of the lake is both aesthetic and recreational. The lake became a local legend because of the largemouth bass and catfish population. To date the record catfish is 36” and the record bass is 26”. In the last decade there have been efforts to improve the quality and appearance of the lake. The main complaint was the filamentous algae that floated to the surface, it looked very unsightly, and smelled accordingly. Starting in 2005, yearly attempts were made to try “low cost” improvements using for example, Aqua-pucks, dye, and hydrogen peroxide. These attempts proved to be too little too late.
In 2009, a bubbler system was installed consisting of 10 bubblers and two 1 hp. air compressors. This reduced the algae somewhat, but resulted in an explosion in weed life. Within two years the lake was choked with weeds.
Boating became almost impossible and fishing very frustrating. In 2015, we introduced 135 triploid grass carp. No improvement was seen by 2016, and 100 more carp were introduced. In the winter of 2017 mechanical weed control was considered, and a number of proposals were obtained. However, this left us with the same hesitation. Mechanical removal seemed to be a short term and expensive solution.
We investigated dredging as a solution and found the cost and permit process to be prohibitive (due to it being a protected water way).
We decided to consult with our local engineer McGoey, Hauser & Edsall Consulting Engineers, D.P.C. (MH&E) They recommended we try biodredging. Everyone agreed that the water quality itself was not the culprit, but the 8-16” of sludge on the bottom of a very shallow lake. MH&E referred us to DeAna Vitela-Hayashi of AquaBio Environmental Technologies.
In the fall of 2017 we submitted water and sludge samples. AquaBio instructed us on how to set up our flatboat for applying the AQUATIC SLUDGE REMEDIATION & PROBIOTIC FORMULA. DeAna of AquaBio sent us a monthly schedule based on the feedback we gave regarding lake appearance and sludge depth. The schedule consisted of weekly applications of the formula. Some of which was applied to the surface and some to the bottom of the lake. Each application took 2-3 hours.
Figure 1. Bellvale Community Lake Bathymetric Map
The first month we saw almost no noticeable change. Then by month two things happened, as is noted in the table below.
Table 1. Depths (inches) at different locations in Bellvale Community Lake on commencement of treatment and at subsequent sampling dates.
Not only did the sludge layer reduce tremendously the appearance improved, and the weeds died off almost entirely. We do get a daily bloom of duckweed, which the carp and catfish attack with vigor. A plan is in place to submit sludge samples for nutrient content to compare with the samples taken a year ago.
Recommendations and Conclusion
Our plan is to continue using AquaBio and the Alken Clear Flo products and further reduce the organic matter and nutrients available to the algae and the plants. Next year our efforts may turn more upstream to treat the inflow. We will continue to add dye as needed to block light from penetrating the bottom of the lake. The carp still have plenty of work to do along the edges of the lake. Our children and teens are back on the lake happily canoeing and angling for the “big one”. Visitors are treated to a beautiful body of water as they drive into our community.
AquaBio’s team can help with your problematic site, please look at our alternative methodology before you decide to mechanically dredge.
Call 424-456-4591 and talk to our team about biological dredging options and using our products and services for your site.