The Tennessean newspaper reports the aicraft slid off taxiway T4, which connects the SW (irony?) corner of the ramp to the hold areas for 20C, and then parallels 20C/02C on the east side.
There
is a drainage sump, and even a creek (or crik, as they say in Tennessee - Sims Branch) along that taxiway near the ramp. Check google satellite imagery.
[Edit - also a ditch/depression where the Murfreesboro Pike highway tunnels under T4, S, and runway 02C/20C, further south]
Plane skids into ditch at Nashville International Airport, 8 injured
https://www.google.com/maps/place/Nashville+International+Airport/@36.1304523,-86.6751453,1191m/data=!3m1!1e3!4m2!3m1!1s0x0:0xaaf78a5d943ea27b!6m1!1e1