When you replace the soil over the septic tank, mix in a large bag of
peat moss so that it will hold more water in the soil.
Also, if the thing still browns out over the septic tank because the
tank is not set low enough, and the peat moss does not help, you can
always go to plan B which is to actually build up the soil a little
and make a small flower bed right over the tank. That way you can add
another foot of soil and instead of groaning about the browned out
area can plant a bed of ornamental grass or something. Just put a
little stake or something in the soil so that you always locate the
opening to the tank when you need to have it pumped. It makes no
difference whatsoever in the scheme of things if you have to just dig
down a bit more to get to the tank cover opening, and if you had to
disturb a piece of sod with some perennials planted in it, then just
do that and put them back in. It only takes an extra three minutes.