Typically you can only depreciate improvements like drip irrigation, pivot irrigators, wells and pumps, the groundwater supply itself, buildings, etc.My background: we syndicate, operate, and own outright large winegrape vineyards in Texas.

The age of the house, the location/market, expansive soils, ground water levels, and many other things can affect the answer. 

Honestly there isnt much the builder can do to get rid of ground water . 

I've been an environmental lawyer for over 35 years and have encountered only one home tank that exceeded $100K and that was an extraordinary situation where there was pressurized piping with a diesel tank and where the consultant committed fraud by over-excavating the site. most home heating oil tanks will range from $10K to $20K with costs approaching $50K-100K where groundwater is impacted and used for drinking purposes.   

On the flipside if you had a marina that did not service boats  in any way you could always put condos next to it just be very very careful about ground water quality soil quality previous spills all kinds of fun stuff and by fun stuff I mean everything having to do with the boat is toxic  in the eyes of the Manatee huggers and if you're going to build on it you may have to clean it up first .

They want me to drill a well which would cost at least 60k - if I'm lucky and can actually hit the groundwater.

Usually, only heating oil USTs associated with commercial properties are regulated. thus, most home heating oil tanks may and often are taken out of service without any oversight from state regulators.Second- the rules that are in effect for heating oil tanks are usually less stringent than for diesel or gasoline tanks since heating oil tends to thicker and may not migrate as far. as a result, many states just require tanks to be cleaned out. they may also require the tanks to be filled with sand or concrete to prevent collapse. but sampling may not be required.Third- the key to determining if a tank has leaked is to collect soil samples from around and beneath the tank (groundwater also if groundwater is shallow). another quick way to see if a tank is leaking is to "dip" it with a stick that has a paste that turns color in the presence of water. generally, if water has gotten into a tank, it can mean oil has leaked out of the tank (although a certain amount of condensation may be present even where there is no leak so this is not a infallible test).Fourth- if there is currently a tank in the basement, ask the owner if there used to be a buried tank (a/k/a underground storage tank).Fifth, If the tank has impacted the soil, the cleanup generally varies from $25K to $50K though the costs will depend ont he depth of the contamination. i once had a home with a heated pool that had pressurized piping and used diesel. the contamination went down 40 feet and the consultant went crazy excavating the soil to the tune of $400K!!!

The realtor is saying we can get a price adjustment but a soil delineation/ground water testing has to be carried out for me to come up with even a conservative number to protect me.

Example: We recently had a plot of land under contract and while walking the land discovered that a corner portion of the lot was wet from the ground water runoff of a new development above it.

If its real bad you may need a hydrogeologist, a person who studies the ways that groundwater (hydro) moves through the soil and rock of the earth (geology) or a hydrologist, someone who studies surface water.You will most likely want a HAZMAT spill team to asses the situation and provide clean up.