If your quarterly water level elevation data is in a single column, then one can use Tables=> Summary to calculate the mean of the quarterly averages.
If your data are laid out with each quarter in a separate column, then you will need to create a new column that has the formula
mean(qt1,qt2,qt3,qt4),
where qt1-qt4 need to be replaced with the actual names of the columns have have the average data.
Jim