Channel Bleeding with ADG732
Between each multiplexer input line (whether on or off) and the common drain (output) there is a capacitance of around 150 pF. This is round-about stated on page 4 of the data sheet - it talks about On Switch Drain, Source Capacitance of 350 pF and it will be similar when the switch is off because internally it's a JFET or MOSFET. It might be as low as 100 pF when off but, for a ball-park figure I'd go for 150 pF.

So you have 31 unused channels all injecting a bit of charge through these 150 pF capacitors and, on the common drain pin you have a virtual earth amplifier responding accordingly. I understand it's this way because you are trying to amplify charge and convert it to voltage.

Do you start to see the problem you have? All 31 together can be regarded as a single input with a capacitance directly to the D pin of nearly 5 nF and this will potentially be a significant noise source and cross-interference source.

Also, on your schematic I don't see where your 0 volts comes from.