Rain and snow can put a damper on most land developers’ plans.  There are situations, however, where too little of the wet stuff can actually slow the development process.  This happens where the proposed house lots or commercial sites will use septic systems and there is concern about lots of clay in the soil or a shallow water table.  The reason for this has to do with the way a septic system is designed.

Septic systems are designed according to the soil in which they will be installed.  You have to answer two questions when you evaluate soils for septic systems.  First, how fast will liquid percolate, or seep, into the soil?  Second, how deep do the good soils go beneath the septic system’s gravel trench or sand mound? 

The wetter the soil, the slower liquid will pass through it.  This is why soil must be wet all the way through before you record a percolation rate.  In most soils, this is done at the time of testing by letting some of the water used for testing soak into the soil  Soils with a lot of clay, however, may need weeks of rainfall or snowmelt before you can tell how they will really function during the wettest time of the year.  That’s because clay particles swell very slowly over time.  While a test in these soils may appear to pass during the summer months, it may fail miserably in March or April.  It would be a mistake to install a system based on such a misleading test.  It could lead to a failing system and a major headache for the property owner.

Concerning the depth of good soils, Maryland regulations recognize that at least four feet of good filtering soil are needed to clean up the sewage effluent (the liquid portion of sewage) from a septic system.  When you have four feet of good soils between your septic system and the water table, groundwater should not become contaminated.  In much of our county, water tables are rather deep.  However, in low areas, near streams, and in certain types of soils, water tables are much closer to the surface and seasonal fluctuation can present a problem.  It is not unusual for a water table to rise five or more feet in the winter and spring.  Here, too, soil evaluations conducted in the summer or fall will not reveal the full picture and poor timing could also lead to system failure. 

Based on these concerns, Maryland regulations require that percolation tests “be performed at the time of year when the highest water table can be expected at the on-site sewage disposal area.”  Where water tables are deep and the soils are not very clayey, tests can be run year round.  Other sites, however, are restricted to the wet weather testing season.  Generally, this time period is considered to be February 1 through April 15.  These dates are fine-tuned each year based on water table levels and how much it has rained or snowed.  In a very wet year, testing may start earlier than February and/or go beyond April 15.  On the other hand, if there has not been enough precipitation, the wet weather season could be shortened or cancelled.

If someone wants to run percolation tests on their property, they must have a surveyor send a plan of that property to the Health Department.  Since the wet weather season is so short, there is a deadline for getting these plans in.  If a developer or property owner wants to have their property evaluated during the wet weather season, their surveyor must get the proper plans to the Health Department by February 28 of the same year. 

Developers should submit their plans well before the deadline.  This will help in two ways.  First, if there are problems, there is time for the Health Department to review the plan and send its comments to the surveyor.  The surveyor can then send a corrected version of the plan back to the Health Department.  Also, it is not unusual to encounter problems with wet weather soils.  Sometimes things do not work out as everyone had hoped and it is necessary to regroup and try again.  So a second benefit to getting plans in early is that testing can be scheduled as early as possible in the wet weather season.  If things do not go well, there should be time for the surveyor to revise the plan, get it back to the Health Department before February 28, and get approval to run more percolation tests. 

Anyone who has questions about wet weather soils or the wet weather testing period can contact the Carroll County Health Department’s Bureau of Environmental Health.