Use differential lines with suitable line drivers/receivers, plus some protection devices (transorbs, GDT or varistor). I would grab a cheap Arduino Nano clone and put it at the reservoir end to read the sensor data and translate it into serial messages sent down the differential line as RS485/422. The Uno would have a shield with a line receiver to convert back to single-ended, fed into the UART for further processing/display. Twisted pairs are nice, but RS485 will work without them if you keep the baud rate low enough.