Had the same problem so I asked one of my friends who owns a landscaping business. It's because the soil above the septic tank is shallow and heats up killing the grass. it also won't hold water. It would come up greener faster than the rest of the yard in the spring then die off when it got hot. I ended up putting a flower bed there to disquise it.
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Posted in reply to: Septic Tank by Splicer Life 4me