The Israelites made an idol that they worshiped as their god. This idol was made of gold. Moses destroyed the idol and ground it into gold ash
And he took the calf which they had made, and burnt it with fire, and ground it to powder, and scattered it upon the water, and made the people of Israel drink it.(Exodus 32:20)
The reason Moses made the people of Israel, who worshiped the golden calf, drink the water with the ash was because of their sin.
Then I took the sinful thing, the calf which you had made, and burned it with fire and crushed it, grinding it very small, until it was as fine as dust; and I threw the dust of it into the brook that descended out of the mountain. (Deuteronomy 9:21)
By taking the golden calf and destroying it, by making it into ashes, there is the idea of from ashes to ashes; whatever you do as humans is worthless. You have no everlasting value if you rely upon your own resources, for whatever you create is lifeless and powerless to deliver you from the bondage of death. As a human being caught up in sin, you have no hope, so why do you try in vain to usurp the purpose of God.
The purpose of God shall be brought about because, unlike humans, our Heavenly Father has the last say and lives outside of time. Therefore, whatever happens, it is only the purpose of God that shall prevail. This is explained by Moses in Deuteronomy as he contextualizes the reason why he made them drink the water with the ash. Moses says:
“Do not say in your heart, after the Lord your God has thrust them out before you, ‘It is because of my righteousness that the Lord has brought me in to possess this land’; whereas it is because of the wickedness of these nations that the Lord is driving them out before you. Not because of your righteousness or the uprightness of your heart are you going in to possess their land; but because of the wickedness of these nations the Lord your God is driving them out from before you, and that he may confirm the word which the Lord swore to your fathers, to Abraham, to Isaac, and to Jacob. (Deuteronomy 9:4–5)
The brook that Moses cast the ashes into, and from which he made the Israelites drink, was that which came from the rock on Mount Horeb.
And the Lord said to Moses, “Pass on before the people, taking with you some of the elders of Israel; and take in your hand the rod with which you struck the Nile, and go. Behold, I will stand before you there on the rock at Horeb; and you shall strike the rock, and water shall come out of it, that the people may drink.” And Moses did so, in the sight of the elders of Israel. And he called the name of the place Massahand Mer′ibah, because of the faultfinding of the children of Israel, and because they put the Lord to the proof by saying, “Is the Lord among us or not?” (Exodus 17:5-7)
This brook came about because of the people’s discontent and unbelief. The Israelites were given to drink the water of contention that was proof of God’s provision (although the unbelievers still did not believe). This water flowed from the rock at Horeb and the Rock we are told was Christ.
I want you to know, brethren, that our fathers were all under the cloud, and all passed through the sea, and all were baptized into Moses in the cloud and in the sea, and all ate the same supernatural food and all drank the same supernatural drink. For they drank from the supernatural Rock which followed them, and the Rock was Christ. Nevertheless with most of them God was not pleased; for they were overthrown in the wilderness. (1 Corinthians 10:1–5)
The Israelites saw that the sinful thing was thrown into the water. They drank the supernaturally delivered water and, in doing so, those who had the eyes of their hearts enlightened would have recognized that which God provides give life, whereas that which humans create is of no real value in the final assessment. Yet God will wash the sin away and provide life for all who accept the truth of His Word.