Spoil Bank Succession & Constructed Wetlands
SPOIL BANKS AND THEIR PLANT SUCCESSION
As discussed earlier, spoil banks are piles of soil that result from the creation of a canal, deepened channel, barrow pit, or some similar structure. They are unnatural and always have an effect on the area where they are situated. Hydrology (previously discussed) aside, they allow some plant species to invade broad habitats where they are normally absent.
Some years ago, a study (unpublished by Monte, as cited by Bahr and Hebrard, 1976: 73) attempted to quantify how plant succession would differ on spoil banks created in four different wetland habitats. The following shows the chronology of invasion by various plant species.
1 yr - ragweed, goldenrod, Eupatorium, tree seedlings, peppervine, deer pea.
4 yr - 20 ft trees: willow, cottonwood, swamp red maple.
10 yr - oaks, etc.
1 yr - very diverse: groundsel, willow, bulltongue, goldenrod, aster, etc.
3 yr - 12 ft willows, 6 ft groundsel
5 yr - herbs shaded out
15 yr - 20 ft willows and swamp red maple; understory elderberry, Eupatorium, groundsel
30 yr - hackberry, willow, toothache tree, Chinese tallow
1 yr - wiregrass, groundsel, and open ground
3-5 yr - wiregrass, groundsel, salt grass
10 yr - groundsel dominant, grasses gone, a few toothache trees
20 yr - groundsel, wax myrtle, toothache tree, elm, goldenrod
30 yr - hackberry, toothache tree, willow
1 yr - 50% oyster grass, rest open soil
5 yr - wiregrass dominated, spotted with groundsel
10 yr - groundsel and marshelder 7 ft tall, grasses gone
30 yr - hackberry, toothache tree, elderberry understory
When transects of bottomland hardwoods were compared to vegetation on spoil banks after 30 years of growth, there was a 34% similarity.
Constructed wetlands is a term being thrown about with some frequency recently in the Greater New Orleans area.
Some ten years ago, most of that discussion centered on breakthroughs at Stennis Space Center by Dr. Bill Woolverton regarding his using plants and gravel beds to treat sewage. Dr. Woolverton was researching possible ways to handle human waste in outer space when space stations are the "places to be." The non-space applications of his research were obvious - handling sewage in difficult situations such as camps in the lake, dwellings in the marsh, and other rural situations.
There have been many advances in this field and uses have ranged from one room facilities to handling the sewage for entire cities. Major conferences have been held and textbooks written.
So, what's the story - why might they be important in Greater New Orleans? The concept is that Mother Nature generally does things better and more efficiently than we do, so we should once again try to be "technology mimics," i.e., use our engineering and scientific capabilities to do what She does naturally! For eons, animal waste and other organic products have washed into bodies of water and have been broken down to the simplest elements and been absorbed back into nature. This occurs when there is a dense mass of aquatic plants that slow the movement of the organically laden water and provide an enormous surface area of plant tissue and associated micro- and macroscopic life that reacts with the organic material and reduces it to simpler forms.
As humankind began to settle the wetlands of coastal Louisiana, our activities began to disrupt and destroy the natural stands of marsh vegetation. We did it through direct (digging it up, piling soil and solids on top of it, etc.) and indirect (allowing salt water intrusion, polluting the water with herbicides and other chemicals, damming waterways and thus allowing water levels to rise, etc.) destruction.
At the same time that we removed the marshes that are so vital to the health of our waterways, we also increased the amount of harmful substances we put into them:
1. hydrocarbons from exhaust and leaks that collect on streets and wash in after rains;
2. Oil, antifreeze, etc. that we pour down storm drains;
3. Herbicides and pesticides that we apply to our lawns that wash into the drains with rain;
4. Pet poop from the yards;
5. Leakage from broken sewer pipes;
6. Deoxygenated water from our drainage canals;
7. Grass clippings from lawns that enter the waterways and use oxygen as they rot and result in more organic matter in the system; and more.
Guess where we go from here? We have to admit that we probably will not be able to intentionally reduce the human population. There are, however, two things we can do:
1. Reduce (drastically) the non-natural things we put in our waterways. Fix leaks in our vehicle's systems. Recycle oil and antifreeze. Use fewer (or no) herbicides and pesticides and, when we do, use them sparingly. Bury our pet poop six inches or, better, flush it down a toilet (I know, this sounds ridiculous - but leaving it out to pollute our swimming areas is not acceptable). Demand that our elected officials tackle the difficult and costly problem of a weakened and dangerous sewer system. Stop allowing human produced organic material (such as grass clippings, trimmings, etc.) to enter our waterways (it is better to compost them and reuse them around your dwellings).
2. Refurbish the marshes. In Greater New Orleans, that translates to constructing man-made wetlands at the openings of our drainage out-flow canals.
This reestablishes the wetlands that were naturally along our coastline and allows them to once again tackle the problem of breaking down our organic waste to simpler compounds that are less harmful to the environment, and in most cases provide many useful components of the overall food chain.
There are quite a few constructed wetlands projects being seriously proposed in our area. There was a test plot funded by EPA through the Lake Pontchartrain Basin Foundation to be located on the Bonnabel Boat Launch in Metairie (this has been discontinued). Another is the proposed barrier island with marsh behind it to be constructed between Lake Front Airport and Paris Avenue. Another is to divert all runoff from the New Orleans International Airport into the LaBranche Wetlands.
This type of project should be part of the solution to a healthier future for our lake and adjacent wetlands.