Question
What would be the most likely causes for dying grass directly over the individual drain legs of my septic systems drain field? The rest of my lawn appears to be very healthy however there are very distinct brown lines in the yard which clearly define the location of the drain piping. I am thinking that soil testing may be in order but want to make the fix logically and not by using a time consuming and costly "shot gun" approach.

Answer
Hi Carl, If the ground is wet there, I suspect either root rot/fungal problems or a toxin that has been added to the lines (root killers, drain cleaning agents, etc).  Either way, I'm concerned that your seepage is coming to the surface.  Check your pH.  It's a cheap test and may be your answer.  Dig a little deeper and see if the soil is blue or smells like sewage.
If the soil is dry, I have seen where the lines were covered with a different soil than the original fill dirt.  Sandy mixes are easier to work with but also dry too fast and I've seen grass die from drought.  Jim