I am looking for ideas or methods to go about examining the extent or existence of a particular isolated basin. The basin occurs on a tract used for mixed hay production since the early 1980's. The isolated basin in question is currently identified as a one acre, Type 1, PEMA. The soil survey depicts non-hydric loamy soils. The basin is directly adjacent to a gravel parking area (1+ acre,1991 construction) elevated 12" above the identified wetland basin. The gravel parking area overlaps/abuts the wetland creating a 90 degree corner within the center of the wetland.

I am looking for ideas or suggestions to determine A: whether or not the identified wetland basin existed prior to the 1991 gravel parking lot or is the basin a result of the gravel parking lot, B: if the basin did historically exist then the extent or size, C: some combination of A & B (a smaller basin existed and the parking lot has/is contributing hydrology).

Office methods include aerial photos, topo maps,or the National Map (http://www.nationalmap.usgs.gov).

The soil survey doesn't show hydric soils, but that doesn't mean the basin doesn't have them. If the basin doesn't have hydric soils and the vegetation is wetland vegetation, it's possible the basin hasn't existed that long. Also, if the basin was dug out you wouldn't expect to see a topsoil layer at all. You could also try digging through the gravel parking lot to determine whether there is hydric soils below, although that would be difficult and destructive.

Mr. Hodapp,

Are you sure the PEM A is actually a wetland? Who says so? When was the determination made? Was there a wetland fill permit issued for the parking lot? If there was a permit, this should show the historical extent of the wetland. On the other hand, you are probably trying to determine if a permit was required.

The soils might indicate how long the area has been a wetland. If the PEMA area has soils that are newly exposed and becoming hydric, you might surmize that the wetland is new and possibly created as a result of the parking lot construction.

Thanks for your responses. The area was determined to be a wetland by the local regulator in 2003 for state wetland regulatory purposes. There was not a permit issued for the neighboring parking lot. The soils appear to generally show hydric characteristics now, but how long does it take certain characteristics to develop? Aerial photos and slide review essentially show 25 years worth of hay field with no clear wetland signatures present. No wetlands shown on NWI. The soils appear to generally show hydric characteristics now, but how long does it take certain characteristics to develop?

You mentioned that past air photos don't show signs of a wetland here. Do current airphotos show a clear difference and you can now clearly see a wetland where none was evident before? If so, that would strongly indicate that the area has become wetter and the parking lot would be the most obvious place to look for the cause of the change.

You can try to compare the soils inside the wetland to the soils outside the wetland in order to determine if the wetland area is simply a borrow pit. Things that indicate that it is a borrow pit would included:
Lack of organic duff in the wetland, while the upland hay field has duff.
Lack of any organics in the soil of the wetland.
The soil profile in the wetland is the same as the lower soil profile upland area, except the upper couple feet have been removed.

Things that would indicate that the wetland has been there and the depression is natural, not a borrow pit:
Accumulated layer of organic material in the wetland. At least as much as in the surrounding upland. Organic accumulation rate are quite variable, but it generally takes a long time for the pure organic layer to develop. In the neighborhood of 100 years per inch. Much less organic material is required for the A layer, or the layer with both organic and mineral soil.
A large tree in the depression that is older than the parking lot. Any trees would have been destroyed in the excavation process.
Redox features and mottling that are distinctly different than the redox features near the water table of upland areas. This would indicate that the redox features formed in the wetland are not simply exposed subsoil redox features.

You might also ask your local soil survey people (NRCS in the Dept. of Agriculture) to have a look at the site. A short site visit would probably be free.

To determine if the parking lot caused the area to become wet, you will need to investigate the changing drainage. Try to determine where the site drains now compared to where it used to drain. If the parking low area was surrounded by higher ground, the area probably did not drain uphill. If the hayfield was the lowest elevation path around the wetland, then that was probably where it drained and still drains. In that case, the parking lot did not likely cause the area to becoming a wetland.

I'm no soils guru, but the popular info re: rate of hydric soils development taking tens if not hundreds of years may not be correct for every soil/landscape position.

I had a student once take samples of highly oxidized (red) upland soil and subject it to several treatments (temps., glucose, anaerobic bacteria, nitrogen atmosphere, and combinations) in the lab. After only two weeks the chroma change was from 6 to 2 for the combination. So, at least changes in chroma in the lab can be fast if the soil remains anaerobic and there is substrate for bacterial respiration. Results were different when repeated in situ in the field because soils could not be maintained under water-saturated anaerobic conditions.

Of course chroma is not the whole story re: hydric soils.

There are publications by soil scientists that touch on rates of hydric soils development, I think.

http://www.epa.gov/reg3esd1/hydricsoils/pdf/allabstracts.pdf

See next to last abstract in the above.

Two more thoughts:

1. Mixed hay can include a lot of different species. If the area has been in wheat, then it is unlikely that it was historically wet. However, often mixed hay means saltgrass, aleopercus and other fac to facw species. In my area, "mixed hay" is often synonymous with "subirrigated pasture", which predominantly turns out to be seasonal wetland.

2. To resolve a gravel parking lot issues, I would get a trackhoe out to dig a hole. A good operator can separate layers, replace in order and tamp down effectively so that no net disturbance remains. Cost is generally cheap and the method is effective; gravel lot owner acceptance is likely to be much lower. You do need to make sure though that you dig deep enough and that you can separate out mixed and buried soil layers.