Water bill 105,000 gallons/2 months vs history of 6,000-7,000 gal

[S Harper]

Providing lots of detail below, but the TL:DR - water usage 15 times the usual bill. No leaks found anywhere (including running toilets) and unsure how they would go through this much water accidently. How would you split the bill with good tenants?

I have 2 tenants in a house with no pool, no sprinkler system, and have lived there 9 months. Water is billed every two months. Their first 3 bi-monthly usage bills were for 6,000, 6,000, and 7,000 gallons. Their usage for Feb & March was 105,000 gallons. Tenant called me extremely worried when he got his bill.  Usually $80, it was $678.58. My question is - how would you share the cost with the tenant?

They are good tenants, one person works from home and said he never heard anything running, and we have done everything possible to rule out any leaks. 

City said it had been a physical read because the meter transponder was non-responsive and thought it was probably an error by the meter reader. They sent someone out to re-read the meter and confirmed the read at 105,000 gallons.

City walked tenant through basic leak check. I also hired a plumber out to do a leak detection visit and nothing was found. The water main, toilet flappers, faucets, water heater and dishwasher have all been replaced within the past couple of years. 

City and plumber both verified there was no meter movement when the water main was turned off inside as well as when it was open and everything was turned off inside. A follow up read at 1 month indicated water usage at 3,000 gallons (same as a regular non-summer month.)

I pulled usage records for the past 2.5 years and different tenancies - in the summers there were 2 bi-monthly bills at 20,000 gallons, almost all the others ranged 4,000-10,000 and a couple at 12,000.

The city said their leak detection software doesn't show any alerts for the house (24+ hours of continuous water running) but won't discuss if that was affected by the transponder being inoperable or how long it was not working before it was noticed. Can't provide any detailed water usage (days, times of peak usage) info again because of the transponder.

I sent an appeal letter to the city and the only thing that they will do is re-invoice the 105,000 gallons at 1st tier pricing, which lowered the bill from $678.58 to $511.35. Previous bill for tenants was around $80, so difference of around $431.  There is difference of opinion with the partners as to how much to ask the tenant to pay (one says 50/50, one says tenant bears most of the responsiblity).

Look forward to your input. Thx.