an "overburden aquifer" is a local aquifer trapped in "overburden" material i.e. in sediments that are above (a burden over ...) a deeper aquifer - a "bedrock aquifer";

an "overburden aquifer" well may be underground but it's also above the deeper bedrock aquifer, so it's also called a "perched aquifer"

the "deep" bit of "deep overburden aquifer" could be about how deep underground is this "overburden aquifer"

"Private Rural Water Supplies
...
Drilled Wells

A drilled well consists of a small-diameter casing ranging in size from 10-20 cm (4-8 in.). Drilled wells can obtain water from both overburden and bedrock aquifers. Overburden wells include those constructed into overburden aquifers (the geological materials above bedrock), composed typically of sands and gravels, but may also be constructed in silts and clays. ..."
http://www.omafra.gov.on.ca/english/environment/facts/06-117...


"THE ABC OF AQUIFERS
...
PERCHED AQUIFERS
occur where ground water is perched above unsaturated rock formations as a result of a discontinuous impermeable layer. Perched aquifers are fairly common in glacial sediments.
They also occur in other sedimentary formations where weathered layers, ancient soils or caliche (a calcareous layer common in semi-arid areas) have created impermeable zones. ..."
http://www.tafths.org/ourpages/auto/2012/1/19/46927603/abcso...
... above unsaturated rock formations [[ i.e. in "overburden" layers]]

"LES SYSTEMES AQUIFERES
...
soutien d'une nappe

Une nappe est limitée vers le bas par un niveau imperméable. Elle peut être alimentée, ou soutenue, par l'infiltration de l'eau d'une rivière. Une nappe est dite perchée si elle surmonte une autre nappe libre qu'elle peut alimenter par drainance. Par exemple, dans la région de Soisson, les nappes du Tertiaire sont dans des nappes perchées; la nappe de la craie et les nappes alluviales sont soutenues par les rivières. ..."
https://www.u-picardie.fr/beauchamp/cours.qge/du-7.htm