New 2011 EPA Regulations say that even a slow drain in your leach field or elevated Nitrate levels could require replacement of your entire system for $10,000 to $80,000 or connect to the city sewer system.
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UN Agenda 21, Ch. 18 - http://www.un.org/esa/dsd/agenda21/res_agenda21_18.shtml
US Clean Water Act - http://www.epa.gov/oecaerth/assistance/bystatute/cwa/index.html
EPA TMDL (Nitrate Limits for Water Supplies) - http://www.epa.gov/chesapeakebaytmdl/
EPA Mandated Inspections - http://www.marex.uga.edu/advisory/cssmip.html
Wayne is correct. There can be a shared septic system. This typically came up on a family farm, where one of the children build a house on family hand and had a joint septic system in place. Often, this was done without any formal agreement, because after all it was between family.
The problems started to arise when one or both parties decided to sell part of the property. Then either a formal agreement needed to be put into place spelling out how the system was to be maintained and who was responsible.
If a property was sold without a joint system in place, it is possible that the property with the septic system physically on their land could deny the other property access to it. Forcing the 2nd property to install an independent septic.
While not exactly common place, it is not unheard of especially in Western Wisconsin.
In regards to the 2nd part of your question is it recorded; that depends on if an agreement is in place. If a legally binding agreement is in place it should be on file with the county. If not, it may not be as simple. Your best bet would be to call the county land/zoning office to see what is currently on file.
This is a most interesting question.....
Are you thinking of creating this scenario or purchasing property with this existing condition? With permit requirements, inspections, county regulations, I would think this to be a most improbable situation.
But then....I've been wrong before!
Your best resource for accurate information will likely be your county/city code enforcement office.