Since rain water storage pit is an immovable property (other than plant and machinery), it will be blocked under the following entry:
(c) works contract services when supplied for construction of an immovable property (other than plant and machinery) except where it is an input service for further supply of works contract service;
(d) goods or services or both received by a taxable person for construction of an immovable property (other than plant or machinery) on his own account including when such goods or services or both are used in the course or furtherance of business.
This is regardless of whether it is obligatory under any law or not as such exception is only applicable for sub-clauses up to (b).
Therefore in my opinion, no input tax credit may be available.