Question
Hi,
I own a house in the country that has a septic system. The leach field is in the front yard which is bordered with a five foot wall then the street. The grass never stays green there and I would like to put rock or wood chip there.My question is it OK or good to use weed inhibitor with the rock or wood chips on the leach field or can this damage it over time.

Answer
A good, tough question, Jeff.  I cannot put my finger on a study or report of adverse effects of Roundup (or similar products like Crossbow or Cassoron) impacting a leachfield's performance.  Joshua Smyth, an eHow contributor (eHow.com# identifies potential problems ranging from increased cancer risk #EPA says no#, possible risks of miscarriage, ecological damage to amphibians, and hazards to the applicator -- convulsions, loss of muscle control, skin/eye/facial swelling, etc.  

Glyphosate, the active ingredient, degrades quickly into carbon dioxide by microbial activity.  There is some research that suggests OVER application, over time may cause damage to soil microbes; Certain bacteria, mites, and isopods can be affected by glyphosate, and can end up making the soil worse for the species that depend on conditions that the organisms helped create. In one study nine herbicides were tested.  Glyphosate-containing herbicides were found to be the number two most toxic to SOME bacteria and fungi found in soils.  Most of these risks were in cases where there has been SEVERE usage of glyphosate, and not considered standard results.  

(Sorry - I couldn't find the specific microbe reseearch sited in the article below; Feel free to investigate their footnotes if you want to persue it further)

http://openwetware.org/wiki/840.119:Developing_Glyphosate_Resistance_in_Tobacco_(Jaime_and_Hanna)

Septic leach lines are at least 3 feet down, by health code standards, UNLESS you have a transpiration system.  These non-standard systems are easily identified as a single raised mound, covered by evergreen shrubs, usually found is areas with poor soil percolation rates or high water tables. In your case, no mound & covered by nothing but dead grass, so probably not!  

Rocks and wood chips should efficiently inhibit weed growth, and in my opinion, a reasonable amount #per lable instructions) of weed-inhibitor shouldn't cause damage to the system.  The microbes are in the soil, and the active ingediant, glyphosate, breaks down into CO2 relatively quickly, once in the soil.

Bottom line; I'd feel comfortable using it over my leachfield, if used as directed.  Hope it helps ~m