Question concerning "But-for" Causation.

[USC313]

So lately in Torts we've been reading about Causation and the "But for" test. (That is, "but for" or "without" the defendant's negligence conduct, would the "accident/injury" to plaintiff still have occurred? If so, then the negligent conduct was NOT the cause of the injury and hence no liability. If not, then the negligent conduct was the cause of injury).

However, how does "but for" apply when two separate negligent acts are at play. For example, Defendant A's polluted water leaks into plaintiff's pond and kills all his fish. A few hours later, Defendant's B exact same polluted water reaches the pond but the fish are already dead.

Under the traditional "but for" test, this would seem to negate Defendant A's liability.("But for" or "Without" Defendant A's negligent act of allowing his polluted water to reach plaintiff's pond, Defendant B's polluted water would have killed all the fish.). But how can that be so, since Defendant A was, in fact, the cause of injury?

Any insights?

[oscarsonthepond]

Below is the relevant portion of my outline (and then some).  I don't remember all the details of this and I don't know that my outline sheds much light, but here it is FWIW:

ii. Concurrent Causes – multiple negligent Ds
   1. Neither is “but for” because either alone would have done it (Anderson v. Minneapolis R.R.)
      a. Cause must be a substantial factor
         i. This is the only exception to “but for”
      b. Result: Both are liable
   2. One is “but for” – unsure which one (Summers v. Tice)
      a. Result: Both are liable
   3. All are “but for”, but none would have caused it on its own (Hill v. Edmonds)
      a. Result: All are liable
   4. Market Share Liability
      a. Some courts: When many Ds negligent, but unsure which one caused harm to P, each is liable for the %age of Ds damages corresponding to the chances that it was the cause (Sindell v. Abbott Labs)
         i. Rarely used beyond DES cases – lots of unclear issues

[ericptk2000]

I think your hypo falls most closely to the case when neither would be the "but for" because either alone would have done it.  In this case, you know which one actually caused the death; therefore, you use the alternative standard- the "substantial factor" test. 

Be careful with that outline, it says that under the substantial factor test both are liable.  This is not necessarily the case.  The prime example being when a fire is already blazing and heading toward a house and you throw a match on the ground.  That match may lead to a fire, but it is not a substantial factor in the burning of the house.  Therefore, the defendant that threw the match is not liable.

In this case, the hypo you give seems to indicate that Defendant B polluted the water after the fish were already dead and therefore the pollution was not a substantial factor of the fishes death or plaintiff's injury.  It would be different if both polluted the lake and you were unsure which one caused it.  That would probably follow the (Summers v. Tice) example and both would be liable in so far as they could not prove that it wasn't their negligence.  If they could prove it, then they may not be held liable.  Say I drop a trace amount of acid in the lake; whereas, Exxon spills a massive amount of oil, it would be pretty easy in that case to show that my negligence was not a "substantial factor".