Brown grass above a leaching field, especially during the hottest part of the summer, is not uncommon.  The leaching field is composed of a grid of pipes surrounded by gravel with a relatively thin layer of earth on top.  That thin soil base dries out more quickly than other parts of your yard and the grass above turns brown.

Resist the temptation to give this area more water as that will negatively impact the effectiveness of your leach field.  Extra water will lead to an overload to the absorption pipes in the field and compaction of the soil. Compaction will restrict bacterial activity needed to operate on the leaching effluent.    

Your grass may green up again in the damper, cooler, fall weather but if the summer brown period is too annoying the solution may be planting a different grass.  Buffalo grass (Buchloe dactyloides) is an ornamental grass that is far more drought tolerant than most turf grasses and is hardy in our area.  It will turn brown in the winter dormant season but may be mowed like turf grass.  The Missouri Botanical Garden has an excellent tear sheet describing the attributes and drawbacks of using buffalo grass.  

You can also replace the patch with shallow rooted perennials as long as you keep tilling of the bed to a minimum. Here is a link to a Clemson University Cooperative Extension list of plants that are suitable.  Keep in mind that Clemson is in South Carolina so check the plants for zone 6a hardiness before selecting!

Courtesy of NYBG Plant Information Service