A leak coming from the back of your refrigerator can be one of two things. There are only two sources of water in a refrigerator's cooling system -- the drain system from your freezer's defrosting mechanism and the condensation emanating from the coolant coils that refrigerate the inside of your unit. Water leaking from the back of your refrigerator is likely due to a problem in one or both of these systems.
Defrosting Mechanism: Clogged Drain
The inside of your freezer would quickly become covered in ice accumulation from the humidity in the air if your unit did not have a self-defrosting mechanism. Self-defrost systems periodically allow the freezer to warm to a temperature above freezing to allow any accumulated frost to thaw and drain from the unit. The drain is typically located in one of the back corners of the freezer compartment and, if clogged with debris or ice, can cause leaks. Use a warm solution of water and vinegar to clean the drain and free any ice blocks.
Defrosting Mechanism: Clogged Line
The defrost drain runs from your freezer compartment down to your condensation pan, located underneath your refrigerator near the floor. This line can become clogged with food debris, and may need to be periodically cleaned out. A pipe cleaner can ensure that the first few inches of the line is free and clear. Pouring a small amount of water down the drain ensure it drains down to the condensation pan. A clog further down the line may necessitate having your unit serviced, as the line is not typically replaceable by the user.
Condensation Pan: Debris
Your condensation pan sits under the refrigerator, down near the floor. The heat from the refrigerator compressor and motor help to evaporate the water that accumulates in the pan. A dirty pan can lead to overflows and leaks. Unplug your unit and consult your user manual for the proper way to remove your condensation pan; it is typically accessible by removing the front grill on your unit. Clean the pan thoroughly and reinstall the pan before plugging the unit back in.
Condensation Pan: Improperly Seated
If you have recently removed your condensation pan for cleaning or maintenance, you may have failed to seat the pan back in place properly. This can lead to overflows and leaks and, left unattended, may damage your kitchen floor. The pan should fit snugly in place and be level. Check the unit frequently for signs of continued leaking.