I think the idea of an underground greenhouse is fantastic. Mike Oehler has some good designs, but one of my favorites is the one featured on Treehugger a while back

Not sure how well that design would work in a place with high groundwater, though...

Given your relatively high groundwater levels, the trench might become a pool.

Maybe a path at ground level with raised beds on both sides would work better at your location?

I bet it would be more efficient to build the brick wall above ground, facing south, and the translucent walls built as a lean-to. With the sun low in the sky during the cool months, most of the bricks in the trenched version are in the shade the way it's pictured. I doubt if such a shallow, small trench provides much of a geo-thermal effect to provide additional heat inside that structure.

My simple hoop house (no digging & no brick work) allows me to keep harvesting greens, radishes & tomatoes well into November, located 42*N.

The heat capacity of water is around 4 J·g⁻¹·K⁻¹. Concrete is only 0.88 J·g⁻¹·K⁻¹. So a kg of water can store 4x the amount of heat of a kg of concrete.

Even if you factor in the density (1 g/cm³ for water, 2.4 g/cm³ for concrete), the heat capacity of water is 4 J·cm⁻³·K⁻¹ against 2.1 J·cm⁻³·K⁻¹ for concrete. So a volume of water can store twice the amount of energy of the same volume of concrete.