Covid 19 is being found in waste water in China
Does this mean that you could become infected by the virus if you have contact with the tainted water?

It's found in wastewater here as well (and rivers/drainage etc) I guess there's always a chance but you'd have to drink it in sufficient quantities

Well you'd have to immerse yourself in a concentrated mass (preferably with open wounds), so unlikely, but possible.

I think they are detecting viral RNA fragments rather than intact virus.

The test doesn't detect the virus, it detects the RNA from the virus. If a person with a fever and a cough gets a positive test you can assume that the test has found the viral RNA because the person is full of the 'live' virus. If some sewage water or a chopping board at a afish market triggers a positive test then it's most likely that the test found scraps of RNA from 'dead' viruses.

I think whis was being mooted as a possible technique to determine what proportion of the UK populace have had it.

That's different. The test to see if you have had the virus tests for antibodies that your body made to fight it off. These linger on for some time so can be used as an indicator of if you had it in the past. Antobidies aren't the same as fragments of the virus' RNA which can also linger on in the body. Testing for antibodies is more reliable than testing for the RNA.

This stuff was circulating about a month ago.
https://www.ft.com/content/e6ad8aa0-47ad-4ac1-b7f5-3375ab7d7ea6

However, anything mooted by part time PM Boris and his lacklustre government I'd treat with the proverbial pinch of salt.

As I understand it, if you get the virus whilst it can multiply within your body, it is also being killed by your body's defences.  (And if you are lucky, you end up killing all of the viruses.)  The dead virus breaks down and is eliminated in presumably your urine and faeces (as is the live virus ? so be careful cleaning other persons back sides).  Parts of the RNA within the virus has specific ?sequences? which uniquely define the virus.  The broken down dead virus thus releases these identifiable parts into the effluent system, through sewage plants and hence into the environment, usually in rivers. 

Sampling rivers or indeed the outfall from sewage plants can then detect these identifiable parts.  The more people with the virus, the higher the concentration.  But it is far from a one to one relationship and depends on many other factors, the amount of rainfall, the amount of dilution by water usage in the home to name but two significant factors in the dilution effect.  I suspect predicting what proportion of the population has it is may be a step too far, but it does provide a useful indicator of where the virus has almost certainly been.  Hence the recent repeated claim that Italy had it late last year when no one was reporting the symptoms.

The reason that I posed the question is -
Theoretically

If there is a village with some covid 19 viral infections that uses the cave systems below as active sewers, water from which after descending through the system(s) resurging to become the water supply to other villages.

1) if you enter a system that is being used as an active sewer would one become infected?

2) if you entered a resurgence same question

3) would the in habitants in the lower villages be more
    susceptible to covid in fection.

4) same question relating to cesspits feeding into caves and resurging at further locations.

Not from the bits from dead viruses but possibly from any live viruses which might have also been shed by people into the water.  Not quite similar to the concern about being spat on by an infected person, but the same principle.

A single viable virus is extremely unlikely to establish itself in your body, our inbuilt defences are pretty good, it takes a certain amount to overwhelm that (but we don't know what that percentage is & it varies massively throughout the population). Only hospitals or care homes are likely to have those sorts of levels of output...