Groundwater & Basement Water Intrusion in Modesto, CA

That is hardpan. A cemented layer sits close under the topsoil across much of this valley, and once the few inches above it are saturated there is nowhere further down for water to go. It then spreads laterally and sits, because the ground is flat. Adding drainage only helps if there is somewhere lower still accepting water; otherwise the practical move is keeping the standing water away from the building rather than trying to make it soak in.

It affects how long one takes. Tule fog can hold humidity near saturation for a week or more, which means outdoor air offers no drying at all and opening the house makes things worse. Everything has to be done in a closed system with mechanical dehumidification, and readings fall more slowly than the same job would in a dry inland winter. Schedules quoted from experience elsewhere in the state tend to be optimistic here.

It is worth knowing who operates it and what it does in winter. These systems were built to move water to farmland, and many still run on an irrigation schedule rather than a drainage one. A lined canal at capacity is not going to accept your runoff, and in some cases seepage from it raises the ground water alongside. Neither is a reason for alarm, but both change what a drainage plan for the lot can realistically achieve.