I'm not a native speaker of English, but for what I see, there's an error in your first sentence.
This is what I understood:
1. Soil and rocks FROM EXCAVATIONS ARE left in huge tips.
Then, in this case, the translation would be:
1. El suelo y las rocas de las excavaciones se dejan en enormes vertederos.
Talking about your second question, "Washing" has to be translated as "Lavar", not "lavando". This is a common problem for ESL students, because we tend to translate the sulfix "ING" as "NDO", like in "washing > lavando", but it's not always like this. When you're talking about an action, you refer to this action using a Gerund form of the action (verb).
Swimming: Natación. Not "nadando". Ex.: Swimming is good for health.
Cooking: Cocina. Not "cocinando". Ex.: Cooking is like a therapy for me.
Listening: Escucha. Not "escuchando". Ex.: You need to improve your listening.
You might want to study a little more of Gerunds and Infinitives. It will help you a lot in your reading comprehension.
Well, that being said, in this case, "that" is not necessary. The translation would be:
2. Lavar el suelo para aislar minerales contamina los ríos y arroyos.
Just remember, I'm not a native speaker of English or Spanish. So it might be innacurate.
But I hope this helps you, amigo. =]