The only way you could argue pollution of a domestic water supply is if you are using the water in your pool for drinking, cooking, bathing, etc. in your home on a daily basis.
Leaves falling into a pool are not going to be considered "pollution" and worthy of the interest of an environmental agency. Pollution would be limited to release of specific chemicals that are considered hazardous to human health, i.e. if your neighbor was dumping used oil and paint in their backyard and it was seeping into the groundwater or nearby river or stream.
You can sue your neighbor in small claims court for the extra expense of pool cleaning, filters, pumps, etc. and the cost of rebuilding the wall. However, legal actions are typically best reserved for a last resort. If you have a problem and you have a legal venue to try to remedy that problem on your own, the courts expect you to at least attempt to "mitigate" the problem on your own prior to taking it to court.
Expect a judge to ask you in court, "Did you talk to your neighbor, advise them of the situation and ask them to fix it?" "How many times did you contact them about the problem?" "What did they say?" Then, expect to be asked, "Did you attempt to remedy the problem of the limbs hanging over your fence?" "No? Why not?" You will also need to bring copies of any letters you have sent to your neighbor, copies of bills for pool problems related to the leaves, etc.
Bottom line is that judges and courts expect you to avail yourself of all your possible remedies to the situation BEFORE you take it to court. I'd suggest cutting the limbs that are hanging over your property and then sending your neighbor a polite letter explaining that you would like the tree removed because of the damage it is causing to your property and the expense you have already incurred due to the rebuilding of the wall. If they don't respond or don't do anything, then take them to small claims court and request that they pay half of all of your expenses related to the tree.