Arduino analogread: neighbor pin noise on ADC even with big delay
When you do analogRead internal capacitance of the MEGA 2560 are connected to the sensor and must be charge to the right voltage. Because your sensor uses a 220k pulldown your sensor has a large output resistance and recharging the internal capacitances can take some time, especially when there is a large difference between the voltages of two different ADC input pins, but analogRead only connects the pin and the internal capacitance of ADC for a few clock cycles. You can add a small external capacitor (1nF or something like that) to the input pin, that way the internal capacitance of the ADC can be recharged much faster from that capacitor. (See Page 276 in the ATmega2560 datasheet http://www.atmel.com/Images/Atmel-2549-8-bit-AVR-Microcontroller-ATmega640-1280-1281-2560-2561_datasheet.pdf)

Reading a ground pin helps, because your expected value is close to 0. But if the value is not zero or close to that, then reading ground first does not help. Atmel wants sources with 10kOhm or less output impedance. Your source has basically 220kOhm source impedance. (http://www.atmel.com/Images/doc8444.pdf Page 5) It could help to do even more than 2 analog reads, delay between them is not needed and you should use the last value measured in the last analog read.