a large quantity of后面可接可数名词复数或者不可数名词 ，谓语动词根据名词的形式决定单复数。large quantities of 后面可接可数名词复数或者不可数名词,作主语时谓语动词用复数。

e.g. Large quantities of water have been polluted. 大量的水已经被污染。

in quantity/ in large quantities 大量地，成批地。

He can give us a better price if we buy in quantity. 如果大批量购买，他们能给我们一个更优惠的价格。